When to start

Start only after a thourough check up from their Doctor...my advice is prior to high school just use resistance against own weight...(push ups, pull ups, sit ups) and tons of stretching( REAL IMPORTANT TO BE FLEXIBLE)...BODY MASS WILL INCREASE WITH AGE..DON´T RUSH IT...
Secondly, when they start lifting weights...always exercise both sets of muscles e.g. biceps AND triceps...push/pull group...hamstrings and Quads...etc...
go slow...in the case of Football...neck rolls combined with light weights will help the neck
But GO SLOW..and use full extension...contrary to popular opinion...not how heavy you lift...better to have more reps ( with full extension) with Light weights rather than a few lifts of Heavy weights...i.e. let the body develop as a result of GOOD NUTRITION rather than weights and the other stuff at an early age...good luck!