Oh yes, that sounds so familiar! Trying to get ex to help during the summer month was like expecting him to go a whole week without sleep. Always coming up with excuses, telling me he would confirm, but never getting back to me, so would have to make alternative arrangements in the meantime, and then he would tell me that it was my fault he couldn't have them.
The longest he's ever had them was a total of 4 days during the whole 6 weeks! This resulted in me having to desperately sort things out with friends, taking time off around my colleagues and inevitably having to pay for a number of days at the holiday club, which ex never helped with!
The best part is, finally got to the stage where DS and DD didn't need childcare any longer but a couple of years later, ex's DS with his new partner started school and what did he do? Told me kids they needed to help with looking after their brother during school days because he didn't want to pay for childcare!
The outcome of his attitude though is that DD only goes to see him when she is free, and DS is refusing to go all together and doesn't miss not seeing his dad.
As you've said OP, that's what happens when the kids get older and had years of getting used to not being a priority.