I think sweaters made from 100% cotton is best for spring and on cool summer mornings and evenings. I'd say go for white for the girls AND the boys, but give the boys a V-neck and sleevless. We can always sew on those really cute decorative buttons, like cars and zoo animals for the boys and flowers for the girls.
We can just change out the buttons for different ocassions, like eggs and rabbits for Easter, red hearts for valentine's and green clovers for St. Patricks day, red, white a blue buttons for the 4th and so on. This would certainly cut down on the amount of sweaters you need to buy. Just buy a few sizes in the boys and a few in the girls.