I also have in my contract that consistent late pickups can result in termination. Some people will just pay extra. If their spot is at risk they will make sure they aren't late. I would charge double as there is 2 kids.
That how I do it too. Each child, even if siblings, have own contract. So late pick up is per child not per family.
And constant late pick up is instant termination with 4 week fee in lieu of notice as it entirely in their control to avoid.