What law protect children that get hurt in an apt complex gym.

If your child was diagnosed with serious medical injuries, then you should retain counsel to pursue a claim on a contingency fee, which means you don't pay anything unless you win.     

I am sorry to hear about your child's injury and hope he makes a full and complete recovery. There could be multiple responsible parties which the facts will determine. You should start by speaking with a Personal Injury attorney asap, after seeking the appropriate medical evaluation and care. Best, Loren L. Gold, Esq.