Looking at their ages, I'm thinking something like a photo frame (if you covered the glass with something first) and let them glob paint on or collage a few things on with glue!!? Then a photo of them in it. For my dh one Christmas I bought a white bbq apron and did my son's hand and foot prints on it. I do remember thinking I should only have done the hand prints as the foot prints got a bit messy and it didn't look as good as I'd hoped (not to mention he's never used it either!)...
I can't really think of much else

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