I had one set of grandparents because I never knew my late father or his side of the family. Teaching them how to throw the ball, or telling them how to do it (letting them do it on their own- like, “put your foot over here, grab there…”) or something like that? Go for it! I always love having dads at the park. If the parents say yes, then it is your back! This is all a personal family issue, so tell the kids that you don’t know if their parents want them doing it before they are tall enough to do on their own. (And it doesn’t matter if you were male or female.) I really wouldn’t want you helping my kids in this way.
(Even when the last was only 9 months old and climbing everything.) I want my kids strong enough to do the things on their own, otherwise, a nice dad like you helping them up (who might not stick around like you) could lead to my kid getting hurt before they are ready. I have never felt that some equipment was too high if they could get up on their own.
I figured if they were big enough to go across, they were also big enough to fall down. If they couldn’t reach to do it, then they weren’t big enough. Raketemensh, I always told my kids (and the ones at the preschool that I worked at) that if they could get up there and go across the monkey bars on their own, they were big enough to do it.