The Dex is the only problem. Kids are notoriously dextrous, hence why many olympian gymnasts are 12-15 (particularly females). Also, perceptions are actually better as Children (particularly with sound), because the senses haven't been dulled over years.

Also, this is kinda partial to humans. What about already small creatures, like Halflings? By you're rules, a Juvenile Halfling child is actually weaker than a Weasel.