I like dealing with young athletes, because this is a good time to build a base for their conditioning and good habits. This base will help after they reach puberty and are able to use weights. They'll already have both the physical and emotional foundation.
Except for out of season training where older kids are lifiting heavier weights for power and strength, I believe more in doing circuits especially for younger kids. Circuits are a good example of a routine for young children. I used to do these myself when I was younger, even before I was doing martial arts.
What is a circuit? It is simply doing one exercise after another without stopping for 25- 30 minutes
When your young players first starting out doing these circuits, you should have them do one circuit and then rest 1 ½ times as long as it takes for them to do the first circuit then gradually cut the rest time down until they’re doing 25-30 minutes continuously.
If you really want them to get in a lot better shape, after they’re able to do a continuous 25-30 minutes, you can have them try to beat their time. For example, if they’ve done 4 or 5 circuits in 25 minutes, the next time they try to beat the 25 minutes and so on. When they’re unable to beat their time, which will eventually happen, you can either add another circuit or add reps to each exercise and start over.
Example of a circuit
Deep knee bends
If you’re in judo, mixed martial arts or other throwing events, you can do uchikomis with a tube (fitting in on throws) and add that to a circuit.
On opposite days, we recommend doing plyometrics. Doing plyometrics, you can either do isolated one exercise and rest or do in a circuit form. Before doing plyometrics, I recommend that you test their vertical jump, which is standing still and jumping as high as you can and mark that spot. After 2-3 months, re-test your athletes and you will be surprised at their improvement.
Example of plyometrics – box jumps, hurdles, ladder drills, twists (while standing), one-legged hops, in and outs (down the ladder), ball toss. You can see some animations of plyometric exercises on this site.