An analogy that I believe works for all kids:
Tall trees get knocked down by strong winds if the roots branch out instead of down.
My fear with how teaching occurs in church classes is that not enough water will be used in classes that should provide "deep" instruction and/or too much water will be used in classes that should provide basic instruction. I view Seminary as the place for the meat and Sunday School as the place for the milk (relatively speaking). (If all kids who attend Sunday School can handle meat, I say go for it and teach more deeply. Just be sure none of them are choking. Otherwise, provide them with greatly enhanced milk.) My primary concern is that so few kids in many places are attending Seminary - and that Seminary sometimes isn't digging deeply enough and teaching students to digest meat.