Have you asked your ex to do the registration for the activities himself (and pay the fees), as opposed to reimbursing you? I had this issue with my ex not reimbursing me for Kid's one activity, and being out hundreds of dollars until he got around to paying. I finally said "I'd like you to register her for the winter session". He agreed, and paid the up-front cost when he registered. I had to stop enabling his spaciness and disorganization.
Worst case: your ex won't get around to registering and the kids will miss out for a session. Hopefully, the guilt will make him a bit more responsible in the future.
