Here's what we ended up with for Noah:
Mattias's valentines were a toned-down version of Noah's from 3 years ago. We made these in a single morning, because they're simpler (than Noah's old version) and because Mattias is just happier to craft AND because I'd already spent too much time with Noah and his cards. I taped smarties candies on them, which was probably unnecessary: second child lapse in judgement or something.
I thought about doing the same rock theme for both kids, but decided that giving three- and four-year-olds rocks might not be a good idea. Also, Mattias was pretty sure that "you rock" was literal.