OK, firstly why should your ex pay any maintenance when he has the kids half the time?
It sounds like you have the better relationship with the kids, you don't need necessarily to go on holiday, take them to the park, take them on picnics, take them for walks. There are a lot of activities that are entirely free. We could not afford holidays when the kids were little, but they remember their summer holidays as being great fun. We would do things like having picnics in the garden, we would build camps out there, we would go to the park, we would go to the river and spot different birds, we would even play pooh sticks there.
Sounds rather odd, but stop looking at the bad things in your life and start looking at the good.
Oh, and get the kids talking to people that have flown on a plane, they need to know that people don't die just because they have been on a plane.
You are alive, presumably healthy, and you have kids you love and that love you. You are lucky. My sister-in-law has just died, she only found out she was ill in April, had a 8 month out grandson who will now never know her, and a 4 year old grandson that may or may not remember her. Her daughters were in pieces at the funeral. So, yes, you are blooming lucky.