Of course he should get them the things they need for an event he wants to take them to on his weekend. And I'm sure you know that technically you don't HAVE to send them with a thing for his weekends, but if it's a case where the kids would have nothing at his house, I'm sure you'd send their things with them.

The better question is, WILL he buy them what they need? And if the answer is no, then you need to consider who will that hurt. Is this something the kids want to go to or is it just something he's taking them to? If they want to go, then if it were me, I'd suck it up and find a way to get their stuff (borrow, discount store, etc.). If it's just for your ex, and the kids don't care one way or another, then I say send what you want (whether that is nothing or the best you can do) and if your ex wants them to go badly enough, he'll figure out a way to get them the clothes.

You are not responsible for making sure your children are dressed properly for an event he planned on his weekend. If you WANT to send the things the kids have, that is very kind of you, but definitely not required and I definitely would not buy anything, especially if you can't afford it. Don't set a precedent for what you will do in these instances, he will expect it every time in the future.