We're nowhere close to being able to use our Radian in booster mode, but I have a question about it. My kids are light weights and probably won't weigh 50 pounds for a very very long time. If a child fits the booster mode well (belt at middle of the shoulder, low on the lap, not on belly) but doesn't weigh 50 pounds, are you allowed to use it? Any ideas why not (of course the manual says to wait till 50)?