A bit late to the party with this, but these children are really young. They can't advocate for themselves. If @mummy917 's exDH won't play by the rules of common decent parenting, she has only two choices:
- let the children spend half the week neglected
- send reminders (which may be seen as interfering/controlling) in the hope that he is simply inept and with the right scaffolding he will step up
- step in and fill the gaping chasm gaps
Depending on perspective @mummy917 can't win. If she chooses 1. she's knowingly allowing neglect. If chooses 2. She's accused of being interfering and everyone knows he would have been just fine without her intervention. If she chooses 3. She's controlling his time and dictating how he parents.
Personally, @mummy917 I'd do whatever let me sleep at night and ignore anyone that tells you they'd do it differently - they can do, with their own kids.
My general perspective is that when I'm an old lady, with nothing but memories, I want to know that I did my best for my children, with as few regrets as possible. If that means that you buy an extra backpack harness because the alternative is either you taking them out unsafely or his Mum taking them out unsafely, that's better than taking the risk of disaster.
You can't play fairly with people who don't play by the rules.