Day-to-day decisions are the responsibility of the parent with whom the child is spending time, so if that time commenced after school, it's the father's responsibility to organise alternative arrangements for the kids, such as after school care, if he's unable to care for them at that time.
If he was collecting the kids from you, rather than the school, they would be your responsibility until the father picks them up.
With that said, this isn't really a legal dispute the court would want to determine. This is more of a general parenting issue, but it is fair to expect the father to organise care arrangements for them if they're supposed to be in his care at that time.