I feel your pain. My abusive ex would keep clothes he deemed “uncomfortable” and not return them until our kid didn’t fit them anymore. He would also keep clothes he really liked and use them on his time only, never returning them until our kid had outgrown them. When I asked for them back, he would gaslight and say he didn’t have them and I must’ve lost them. He also did this with toys, blankets, hair accessories etc. Now that our kid is older she knows her dad is shady so she takes it upon herself to bring things back. 

It might be worth having it added to the court order that ALL items sent over be returned upon child pick up. 

Why are there so many crazy folks on this planet?

Well, she thinks she's getting custody back at the next hearing. Makes sense.

Next time don't send them with clothing. After all, she has it there.

Where I live she would be in a lot of trouble with family court for keeping kids clothes ESPECIALLY shoes or coats.  

My sisters ex tried to do that crazy non-sense with my nieces and nephews winter coats and the judge had a fit on him!!! 

I don't know your story but I would be having a conversation with family court and if there isn't a mediator I would get one.  

Does his order require him to send clothing with the children? This is a common issue and it's solved by minimizing what moves between homes. They wear one outfit to mom's and they return in one outfit. Even if it's not the same outfit it's much easier to replace then a whole suitcase for. Biomom is a parent too and that means she is resposible for supporting the children when they are with her.

Another option is to send a written request that she return the clothing. If she refuses contact the police and report it stolen.

Mail to BM all the junk clothing she gave the kids.

Both parents must provide accordingly and equally for their kids. I would send 2days worth of play clothing with the kids when they travel and I do mean play clothing. Keep good shoes, and good clothing at home.

Doubt if you will get the clothing back BUT a Judge could put an order demanding that she does.

Again focus on future trips, sending 2 days worth of play clothing. Leaving good clothing at your house.

Sorry about this....

A typical strategy for the toxic morons in the blended family opposition.  The SpermClan used to pull this shit with SS's clothes.

We have always invested in quality clothing and shoes.  It is less expensive to buy quality than it is to by low quality cheap crap ... in the long run.

We would send SS with his clothes to SpermLand for visitaiton and he would often come back in cheezy cheap crap with no sign of his quality clothing or shoes.  We battled SpermGrandHag on this for a number of years.   She responded well to embarrassment and humiliation so that is what we applied in order to get SS's clothes back.  It didn't always work but it worked more often than not.  Always some excuse about her not wanting to send dirty clothes home with him, or that she had 4 GrandSpawn in her home and couldn't keep up with what belonged to which kid, etc....  

Interestingly when we did not recover SS's clothes it would show up in family pics on the three younger Spermidiot spawned half sibs. 

When he got older SS would strip his clothes (Hand them a robe, lead them to the bathroom and not let them out until they handed him his clothing)  from the younger ones so he could bring his things home.  It was so pathetic that when SS got into his mid to late teens the SpermIdiot would steal his clothes.  The most irritating thing is that SpermGrandHag was fond of telling SS that their pittance in CS paid for his clothes. According to her that pittance paid for his clothes, our nice vacations, homes, and vehicles while his half sibs were starving and deprived of the nice things that SS had.  That $133/mo in CS paid for $hundreds of thousands of uneccessary extavigance.

I'm so sorry, it is really so difficult and nervous when your ex made such things. As if they can't buy something beautiful for their kids by themselves.

If the kids ever wear new clothes home BM keeps them at her house. DH always sends anything from her place washed and folded in a bag home. Once he asked if he could get the clothes back and she packed a bag full of dirty old rags that were too small. None of the new stuff. DH stupidly walks in and boasts "look she actually gave us some clothes back" then I was the bad guy for pointing out that she wasn't doing the right thing she was pulling an act. We of course had to fight about how I have a problem with BM. Anyway, kids come over and complain they have no summer clothes. I said "why didn't you bring your new summer clothes back then?" (In front of DH) and the skids says "because mummy said I'm not allowed to wear those ones to dads house" so DH was put back in his place. I ended up texting her myself and mentioned that we will not continue to buy new clothes, shoes etc for them never to be seen again. She acted all nice and said she would send them right away and had no idea. Well what do you know, a heap of old too small clothes come back again that we bought them 3 summers ago and never saw again. Basically all we buy them now is cheap stuff and when they complain we tell them to wear the good stuff home from mums. 
I swear these BMs just think they have right of way to take what they want so they don't have to open their wallets. 

This is a constant fight with the BM. We've just been sucking it up and buying them things that stay here. We've limited the items going back and forth. Next weekend theyre not even going to bring backpacks back and forth.