Such a cute idea! That's a fun way to use up the tons of bags and cards we receive, too.
I suggest that next time you back your pictures with acid free paper (you can just back them or mat them) to give them a layer of protection from the chemicals in the bags. That'll reduce the yellowing and deterioration of your photos over time. They'll be fine for a while, but once your kids are older, they'll start breaking down and discoloring.