I've brought drinkable liquids on board with my child--generally a sippy of water and boxed milk. I put them all in a gallon-size ziploc and pop them in the bins to go through security, so it's easy to see what they are. I've never had problems, but I assume at some point the kid ages out of being allowed to do this?

Also, if your juice boxes get confiscated, you can always buy more after security, I would think.