IIRC, children can use the punching bag (reward object or custom content buyable versions). Older Sims (teen and up) can offer lessons to children on the punching bag.
As for the gym equipment, it is doubtful, as the animations accompanying their use would have to be created in whole as they don't (so far as I know) exist in the game.
For building body skill, children can:
1) Use punching bag (kids will punch only at low body skill, kicking added when higher levels are attained)
2) Use obstacle course (reward object or custom content buyable version)
3) Swim
4) Work out to a stereo or other compatible audio device (radio, phonograph, etc.)
That list may not cover all the possibilities, but they are all which readily come to mind at this moment.
However, whether their mood or biology limits or contemporaneously rejects the use of any of those at any point in time is a factor of the individual Sim.
There are also some custom objects which build body skill and which might make sense for children to be using, such as some of the (non-lethal) light saber scanners here:
http://www.modthesims2.com/download.php?t=268787