Basic fitness training:
It is important that you build a foundation before you do harder and more specialized training, especially for younger kids. Too hard training early on can lead to injuries. I would recommend starting to work with body weight excercises to bring your body in shape.
Always start with a good warmup. Run a couple poles and do some gymnastics (arm circles, jumping jacks….) for the legs and arms. There are a lot of different theories about stretching, but I would do some light dynamic stretches along with the gymnastics.
Running coordination drills:
This is about batspeed but running is important in baseball. Do some high knees, buttkicks, shuffles skips, lunges and other stuff. You can find a lot oft hat stuff on youtube, so I won’t really go too deep into that. You don’t need to do a lot of volume, more important ist hat you do it regularly. Two lanes of 90 feet of each drill is plenty enough.
Sprints: sprints build leg power and of course make you faster. Do a couple sprints about 90-120 feet but only after a good warmup and some coordination work. 5-6 sprints should be enough, don’t get tired.
- Vertical jumps: crouch low and immediately jump up. This triggerst he stretch reflex and builds explosivity. Do 2-3 sets of 5-8 reps.
- Standing long jump series: squat low and swing your arms back, then extend forward and swing the arms forward at the same time. As you land you crouch and swing the arms back and then immedately repeat as you land
The core is extremely important in baseball. It transfers power from the legs to the bat.
- Plank variations. Do normal forward planks, side planks and even some twisted planks like this: https://www.youtube.com/watch?v=hSoWKDfdZwU search plank variations on youtube and don’t do the same every day.
- Crunches: those are disputed too but I think if you don’t overdo them they are still a solid excercise albeit not really baseball specific
- Straight leg pelvic twist: lay on the back, lift your extended legs and move them left of you tot he ground and then right and so on. Great excercise for the obliques. https://www.youtube.com/watch?v=UcTgP9ZYCmMUpper body excercises:
- Push ups. By far the most famous body weight excercise and rightfully so. very good pushing excercise. You can target the triceps more by keeping the hands narrower. Make sure to go all the way down and not too fast. You can increase difficulty by raising feet.
- Pull-ups. Great excercise but not easy to do. for starters you can do lying pull ups on a low bar: https://www.youtube.com/watch?v=0g1_zc4vLi4 until you can do real pull-ups
- Band work: google throwers ten for pointers. Especially important ist he external rotation oft he shoulders for health because baseball targets more the internal rotators.
After your workout do some static stretching to not lose flexibility. Do toe touches, quad stretches, calf stretches and adductor stretches. Also do stretches fort he triceps, pecs, lats and forearms. Losing flexibility can hurt your technique and limit your range of motion causing you to not use the whole kinetic chain. Make sure to stretch the obliques too (like this for example https://www.youtube.com/watch?v=IdIUEUTXP6Y) so that you can increase the hip shoulder separation in the swing.