Honestly, you're only hurting the kids if you keep them from going with their dad. You know his reasons may not be pure, and you probably think it'd "really show him" if you told him he lost his chance, etc etc., but in the long run, thats not what really matters. Plus its only been one day. If he went to the court system, and said "I didn't think I'd be able to take my children due to money issues, but within 24 hours I figured things out and decided I would be able to take them," I dont think they'll see it as him forfeiting, unless he has a specifically court ordered date that he was supposed to pick them up by, and he failed to meet it.
My son's father is the same way. Before our child support hearing, he used to say that he would be taking my son for a specific timeframe, and then last minute give me a guilt trip stating that it was too expensive, and that he would LOVE to take our son, but then he wont be paying support for the month if he does pick him up.
I think the idea of making him pay the price for his actions seems like it would feel good...but in the long run, it just seems like a headache. Let him take them, and have some "you" time. =)