Do not doubt or beat yourself up about how you manage this. I have heard myself begging my ex (out of earshot of the DC's) to come and pick up a DC / be at an event / go halves with a gift that I couldn't afford on my own but they would have had if we had been together, e.g. a bike, a laptop, whatever. It was always because I didn't want them to be disappointed / miss out / feel rejected.
It didn't work and it was even worse when I heard them asking themselves as they got older, and it still didn't work. Honestly my heart used to ache and I cried a ton of tears. We mothers do our best for our children and somehow we still feel the guilt / take the blame when things don't go to plan.
My ex did not want shared care and bought a big house with a big mortgage in the days where outgoings counted for reducing Child Maintenance. I got an access arrangement through a solicitor and he did not follow it.
Though a court can nail down 50/50 child maintenance is based on overnights, it is not really 50/50 so it does not matter if he picks them up at 9pm and brings them back at 7am, it still counts as an overnight, if avoiding paying maintenance is his goal.
Having said that do not let his inaction or disinterest spoil the milestone and key events that you and your children will share. Just be you and don't try to compensate for him. 🌻