Practically any fighting-type, anonymous, but especially members of the machop or riolu lines. Fighting-types often engage in personal exercises to maintain peak physical performance, and they’ll often push their trainers or human companions to do the same. However, members of the riolu line can sense a trainer’s physical limits and will not push a child any further than that, and members of the machop line are often sociable and more understanding of their humans’ limitations in a non-supernatural sense. As such, you can be certain that your children will be able to exercise alongside these pokémon safely.
Alternatively, other fighting-types, such as hawlucha or the tyrogue line, can make exercise fun, which may be beneficial to a child. Either way, you should look no further than a fighter to get someone into exercising.
Best of luck, anonymous!
The Pokédex According to Bill 