Why pay for the kids gym when:

You can go to the park for free?!

I've spent several times paying $7 per visit for my daughter to go the kids gym, just for her to get sick every time.. Don't get me wrong the kids need excersice but they need fresh air too. The positives in going to the park:

No fee
No time limit
Fresh air
Ineraction with other kids
You can join them on the slide!!
Most likely will not get sick!

Cons:
It may be cold
Might have to leave due to rain (but really thats what jackets are for)

I currently cannot think of anything else!
Have a picnic at the park and spend some time in the sun that provides a much needed nutrient! Rather that going to the gym.

If you really think about it, nature provides all you need!