Hi guys,
Regardless of the young athletes age, I would avoid trunk flexion exercises such as situps. Please see this post for more information and references.
http://forums.jpfitness.com/cgi-bin/...;f=14;t=000081
Essentially, any time the lower back "rounds", it can be damaging to the spine. Not something you want to get a 5-year old started with.
Craig