The big problem with trampoline parks is that you have many people of multiple different sizes jumping at the same time. That causes unpredictable movements of the surface that can snap smaller people’s leg bones. Here’s a study published by the NIH that goes into details.

A 6 year old might not hurt themselves on a trampoline, but 10-year-old and a 36-year-old parent jumping with them could cause a lot of damage (and has, many times).