Tattling on a young child in the gym?

Well I don't agree that being annoyed is the proper excuse for what you did, I do think you did the right thing. Gyms are not meant for little ones and he could have very easily been injured by the heavy equipment. I'm surprised the staff didn't say anything to them. So to anser your questioning, your reasoning was a bit crotchety, but overall the result was good for everyone, including the child. If they want a bonding experience they can do it somewhere child-friendly.