Green is a great color -- excellent camouflage. Sand is also versatile. But since visibility is high on your priority list, you may want to consider both boats in orange -- your daughter might be more comfortable knowing where her Dad is for much the same reason as you do. You may wish to swap boats on occasion also, with no loss in parental visability. Finally, you're least likely to get run over by another vessel, especially in low visability conditions.
I don't think color matters much to the fish. To them, it's mostly a big shadow blocking the sun. Against the light from the sky, color seems to vanish!
You'll probably get some color fading, especially in orange, but much of that will depend on how you store the boats. But then, it's like jeans -- sometimes a little fade is cooler -- shows your long standing experience.
UV damage won't be an issue for the kayaks, only for you and your daughter!