AITA for not bringing my kids to their mom for mother's day

I need advice. Me and my soon to be ex wife are going through a divorce. We have 4 kids together and I have primary custody of them. Our kids are 15, 13, 10, and 16 months. Our 15 year old has chosen not to see his mom at the moment. The judge does not force him to see her. Our other 3 kids she gets them one weekend a month. This is the custody agreement SHE came up with. She didn't want 50/50 because she wanted to move to a different state. We live in Oklahoma. She lives in Florida. She gets the kids on the last weekend of the month. Mother's day does not take place at the end of May so it does not fall on her custody day. I usually pay to fly the kids to see her and she wants them to fly here before her custody day so she can see them for mother's day so she would get the kids 2 weekends in May. I don't have an issue with that. What I have an issue with is it is a lot cheaper for her to just fly down here than it is for me to fly 3 kids to her. So I simply ask that she just come down here. She said no and I said then I guess you can't see them for mother's day. I told her she is more than welcome to come here but I'm not paying twice this month to fly 3 kids out there. I'm not made of money. She got upset and said she should get to see her kids for Mother's Day. I said she can. She just has to fly here. Am I being unreasonable? I don't feel I am.

Edit: She will start paying for the flights when she has time to get a little more money. She has only been in Florida less than 6 months and hasn't been at her job a super long time. So I get it. But she can afford a plane ticket for just herself.