Too strong for their stomachs and full of sugar. Water or milk are the best by a long way. Even ds (9) still only has water and he is happy with this as apart from odd occasions it's all he's had. He prefers water to juice anyday.
"Only water" oh yes they say.........unless your DS's simply wouldn't touch the stuff in which case VERY diluted juice was the route we went down with both of them. DS2 (14months) favourite (at the moment) is Coconut and Pineapple juice. He also took quite a shine to the Red Grape juice several months back