If DH won't raise it with them, I would politely do so yourself. In this day and age, everyone has busy lives, and a call first to say "is it convenient?" is the smallest of courtesies.
Also, why not suggest that you and DH pop round to see them sometimes, maybe while your boys are playing sports, if they're not keen to come. It sounds as though they must live nearby so this shouldn't be a time issue. Maybe his son/daughter feels that it would be nice if grandad showed an interest in the grandchild and since he won't visit them, this is the only way to achieve it.

It's courtesy to call first, at least to see if you are at home! Do they have work schedules too? I think it depends on how old the grand kids are too, can they play by themselves? Do they all have to be 'entertained'? I think I'd be OK with them coming around if I knew in advance, had enough food in to feed everyone. I think I might offer to babysit once a month, that's a good way to get to know the grandkids without the complications of the adult SKids being there. As busy as life can be it's a non stop job being a parent, I'd suggest to him that they are his kids and grandkids and nothing anyone says or does will alter that but, PLEASE, DH, let's have some manners and courtesy from them? What if your kids were at camp or on a sleepover and you and he wanted privacy? A call first does not hurt anyone.

Your Dh sees the adults as his 'children' and his door is open 24/7 to them.

Why not go out the next time they arrive? If you have a sports event to attend just smile and say "We were just on our way out. If we knew you were coming we could have changed our plans. Never mind, I'll just run along and you can see your Dad. Honey, I will be home in a couple of hours." I doubt they are busting a gut to see you over their father.

If your DH thinks this is rude remind him you cannot put his life on hold for his kids 24/7. And he is the one being rude by NEVER visiting them in their homes.

This is just plain rude. If your husband won't say anything, you definitely should.

My SD was told a long time ago to call before coming over. She had BM bring her by once without calling. My husband was at work and I just flat out did not answer the door (always locked and she has no key). We have no peephole and the transom is too high for me to see out (I did have a feeling it was her).

I knew, if it was her, she would go around back and peep through the windows so I stayed right where she would be able to see that I was there.

She then later lied to my husband and said she had called, which made no sense. Had she called, I would have told her that her dad was not home so there would have been no reason for her to come over! Or, I would not have even answered the phone, so again, no reason for her to come over. She's not real bright.

I think you need to settle this between you and DH and have DH broach the subject with the adult skids. If you do it yourself, you will just be the "wicked stepmother." Also it encourages alliances between DH & skids against you, and you become the unreasonable one in their eyes. DH should not give the impression that you forced him set this boundary - that is a cop-out and is disrespectful to you. DH needs to make it clear that he loves them and they are always welcome, but they need to respect that everyone needs privacy, even their dad, and they need to call ahead and see if it's a convenient time for a visit.

Are you living in the same house the skids grew up in? That can be an awkward dynamic. They need to realize that they now have their own homes, and they can't just come and go at your house as if they still live there. How would they feel if you showed up on their doorstep without calling ahead?

This situation reminds me of the Joanna Trollope book - "Other People's Children" - it looks at several stepfamilies, including one with adult skids. It was an immense relief to read it and find characters who were going through what I was going through. The character I felt the worst for was the woman who had an adult stepdaughter who had a key to the house and treated it as her own - to the point of cleaning and reorganizing the kitchen, etc.