It’s just the way of life when you have kids. There’s always going to be days where they are ill; have to be collected, you are running late etc etc. It happens. But it doesn’t mean every child needs a parent available 100% of the time, you just make it work through a variety of means
So again, if a single parent can manage it, surely so can 2 perfectly healthy individuals working together as a team? Sure, children don't need someone available 100% of the time and yet, the ex is now available for half of his children 100% of the time and that's apparently fine. No reason for him to change that. OP, on the other hand, should run herself ragged as a single person for no financial gain whatsoever.

I am not advocating that the OP shouldn't work. I am suggesting that much depends on her experience, qualifications, where she lives, availability of suitable work, resources available to her like public transport, a car, proximity of her children's school and nurseries, opening/closing time of school clubs and nurseries, availability of a childcare place when she needs it, the nature of the job she is able to secure when it comes to flexibility etc. etc. etc. A blanket 'I made it work so everyone else' can ignores the reality of many people's situations. Having a child with a health condition that needs regular hospital appointments, for example, can really throw a spanner in the works and is very difficult to overcome - I had to permanently lose half a day to manage that and was lucky my employer offered that flexibility. Future interviews will involve me saying 'my child has a life threatening condition which needs constant management and regular hospital appointments. I might be late some days whilst we sort it but it probably won't happen often but then again, it might. No way of knowing. And I know you advertised a full time job but I'll need some Tuesday mornings off to manage the hospital appointments. Or Tuesday afternoons. The clinic is always Tuesday. Except in a pandemic when the clinic have inexplicably changed to Fridays. I don't want to work part time 'cos frankly I can't afford it so thought I'd chance my arm. Will that work for you?!' Of course, the ex could face similar issues but he has the advantage of being part of a parenting team.
The suggestion is the OP needs to solve the problem but even if she were working flat out, that doesn't mean she shouldn't expect her ex to sort out his side of the bargain and to find some flexibility in his week to ensure his children receive some financial support. He keeps being given a get out of jail free card as far as I can see whilst she is villified for not doing what he's not doing either.