That's a sad situation. It sounds like Mom's having a rough time financially, and is working long hours to try and provide. This isn't legal advice, so you are free to ignore this, but from an outside perspective, it sounds like you think she's being a bad parent, when to me, it sounds like she's working really hard to take care of the kids and make sure they have a roof over their heads.Of course, I buy almost everything they need. It took a few mess ups because when I sent money their mother took it and they didn't get the clothes or shoes or whatever, even Christmas gift cards were taken from them. If we (myself and extended family) sent receipts she often took items back to the store and kept the cash. We now buy online, ship it directly and she'd have to charge it back to the credit card so they actually get what they need now.
She literally buys them almost nothing. I feel like I have to because they need things. It would be nice if she would buy a bottle of lotion or some acne cream but she won't and it costs a fortune to ship so I try to get them to save up their requests and plan ahead and make a large enough order that it ships free.
My daughter tried out for the color guard and made the team, she was so happy and was going to go back in mid-July to start practice but she said there is no need to go back because her mother said she won't take her anyway and she has no way of getting there and home every day.
My family was pretty poor growing up, and there were times when my mom had to take back presents and get cash from them. I didn't really know it as a kid, but as an adult, we've talked about how hard we had it when I was a kid. It sounds a lot like what your kids are going through. She worked very long hours, and we didn't see her that often. I was unable to participate in the band because the summer practices would have required her to take off of work to get me to and from them, and that wasn't an option. We moved a lot because it was hard for her to keep up with rent.
I don't know you or your ex-wife, so really, I could be way off here, but the story just rings some bells for me, and it might be something you're not considering. My father never paid a dime of child support, and it sounds like you're involved enough that you DO, so please don't think I'm trying to blame you. I'm just trying to give you a perspective you may not have considered.