First off, I truly love your heart fro these children and appreciate you wnating to be as active in their lives as possible.
I am going to shoot from the hip here just as a possibility.
As a biomom I only hope if/when someone enters my sons lives as bonus mom that they have their heart where you do, I would be happy to have her join along BUT as a human will admit I will probably have moments of jealousy and want to be their MOM and have to work at not being defensive for the sake of kids.
So guess I am suggesting perhaps if it were made clear to her that she is their mom and you wish to be someone special to them , a bonus mom(Perhaps not called mom)that she would feel more at ease.
I was lucky in my 3 year relationship to have an ex who welcomed me in child's class,bday parties everything but I had told her clearly I only wished to be someone else in the child's world to love her, not her mom, she had that.(some children on here do not have amom or decent one anyway and stepmom is the key female figure)
I see your hgeart in the right place but wonder if you expressed this to her if she wouldn't perhaps be so defebsive about everything and feel she had to run the show, perhaps even compliment something about her mom skills in front of others for her to feel secure.
JUST A SUGGESTION.

Volunteer for school stuff. Teachers and staff always need help with stuff like parties, setting up stuff, reading to the class, etc. BM can do the same and if she chooses not to or can't due to schedule conflicts so be it. As far as I am concerned, you are not stepping on her toes. If you offered and are asked and she did not that is her loss. I am lucky and blessed that I work from home so I can volunteer at my kids' schools quite a bit. There is no reason you can't help out. Kids love it when they have guests in their class/school whether its a parent, sibling or grandparent. Attend PTO meetings and see where you can help if you want to. They have committees that need help with stuff directly with the students and indirectly.

As far as stuff like conferences, our school has a policy that parents who are not together can share the conference time and not be together for it. Example - 30 minutes conference, BMgoes for 15 minutes and DH and SM go for the other 15 minutes.

It can be worked out. I would let BM know you want to be involved but you don't want to "take over".

Good luck! Let us know how it goes.

I am not sure on which days you get the skids or if you have them a week here/a week there kind of thing, so you would have to gear an email/letter to her to account for this. But how about stating something like, I think it is great you have the opportunity to volunteer at the school in so and so's class. The more quality time with a child the better. The children tell us often on how much they love their mom & enjoy any time spent with you. I realize that you work (or if she doesn't ....have a busy schedule) so I wanted to pass something by you. As you know I volunteer at the school already & I was interested as well in volunteering in so and so's class, however, I feel that the choice of days or times should be something you of all people have a pick of first, after all, you are the mother.
If you could let me know what days suit you better then we can arrange something to notify the teacher. If at any time you need to switch a day here or there, please do not hesitate to call me, as I am more than willing to accomodate your schedule changes.

How's would something like that go? This way you are allowing "her" the control of which days to pick and you are complimenting her while doing it....as we all know, control is the issue here with most of the BM's.

I would like to say to you personally that I would die for a stepmom like you to enter my childrens lives. I feel like the more love, guidance and participation you can offer a child the better to prepare them for life in the future. You are a true blessing!!

In the beginning of my step parent role I was allowed to do this as well, but then BM felt like she was losing control over everything when the kids enjoyed being around me....then things went downhill and I was the mean stepmom. Very sad that it had to end up that way.

Let us know how you make out.

Corie

I am a Bio mom as well as a SM and this may come across as harsh to other bm's, but please bear with me.

I know how it feels to have my two year old call his SM "mom" and kiss her goodbye in front of me, to have him call me by her name at times. I know how it feels to hear about all the fun family things my sons were doing with their dad, SM and her three kids within months of us divorcing when they moved right in together. I know how it feels to know that the SM is at the field trip because I couldn't get off work, to have my son turn in a paper in 2nd grade about his home and family where I wasn't even listed because he was over at their house that day and it didn't occur to him to include me. I remember all too well when they chose to give their SM the cooler present because they were sure I would like whatever they gave me, so they wanted to make sure her gift was nicer.

It's hard, sad, your ego takes so many hits.

Which is why that very ego needs to go on a shelf as high up as you can not easily reach it other than to dust it off occasionally for other reasons (work, school, dealing with people out in the world at large.) NEVER never take it down for use against your kids' SM. And if you can just do that, I promise amazing results like mine, I am really good friends with my ex's wife, my sons' SM who they call by her name but have my absolute permission to call her "mom" whenever it suits them.

In your situation I do not feel you need to respect BM's fragile ego other than to refrain from saying hurtful things about her to her children or to her face. Plan your own birthday parties for the kids, and she can be invited as a guest only (DH should make that clear.) If she doesn't act like one but takes over, then I would make clear that she will not be invited next time as calmly as possible and away from the kids.

As much as it sucks, as a SM I would avoid field trips and class parties, other than to offer to send in treats now and then if the teachers would like (that will still show the kids you care a lot.) We also have scheduled our own teacher's conferences, which was kind of stupid on BM's part, because without her or SD present, we could talk honestly to the teachers about our issues, and both teachers in those two years became allies in our quest for custody.

As for larger events like sporting events, school performances etc, you belong there and if your wonderful DH is willing to back you up then by all means go. If she is uncomfortable and can't be mature about it then she may stay home. We used to try not to surprise BM showing up unannounced, but eventually we did stop telling SD for sure we would come to things on her nights with BM because then BM wouldn't bring her to them if she thought we might be there. Once again, she created the problem and secrecy.

Keep being sincere in your motives for involvement in these activities and let BM deal with her own insecurities about it. Not your problem in my book.

Peace, love, and red wine