This sounds like a lot of fun and you're teaching you child that sometimes putting money into savings is hard to do. It kind of goes against the premise of the whole challenge, which is to slowly help you adjust to saving, but it sounds like you are already have it under control.
I'm not a fan of the 52-week challenge the way that it's written. I think that if you can afford to put $200 aside in the month of December, then you should be able to do it the other 12 months of the year. And I wonder how many people who start the challenge with gazelle-like intensity actually push through the tougher months in the latter part of the challenge. I much prefer the idea of just having consistent amounts deposited into savings on a regular basis.
In the long run, I suppose that anything that motivates you to save is a good thing. I just prefer to do it my way instead of relying on a gimmicky plan.
