If you can't trust him then take the "extra" money until they refigure next year. Make sure you use the courts so they can enforce the payment. Plus it's for your kids and with kids there is always something unexpected that comes up.

PLUS you didn't choose the timing for this HE DID. He chose to cheat and when to walk out. Don't let him talk you into a lower payment. He made his bed let him sleep in it.

IMHO of course just realized I mayhaps sounded a bit bossy