Lot's of "if's, and's and but's" here. Sounds like you have cross runways at your field, since you mentioned a crosswind runway. I think the SAFEST situation would be as those above have said, for Dad and Son to wait until noone was flying before they practiced taxiing and ground handling. However, if it had been only an adult out there, then I see nothing wrong with what he was doing, as long as he stayed clear of the runway in use, i.e., the one with favorable winds for takeoff and landing.

The one thing I see that both of you were doing that could have been improved on was for you both to have had a SPOTTER, someone who could focus attention on what was going on around you so that you could continue to focus on your plane and not worry about Junior running over your foot, as well as the other guy being able to focus on keeping Junior out of your way. One other person could have functioned for both of you, since only one was in the air. While it's not mandatory at our field except during sanctioned events, a spotter is worth his weight in glow fuel just for the peace of mind that you can fly and not try to watch more than one plane (yours). They can also be the communicator of your intentions to the others when you have your hands full and might not think to holler "DEADSTICK", or "OH, SH*T!", or "DUCK!!!"

I'm always nervous when young kids are on the field, or even in the pits, during flight ops. Not enough judgement or experience on their part to know what to do in most instances, and you never know just how short a leash Dad is running. It's bad enough for one of us strapping young adults or old farts (take your pick which one I am) to get hurt, but it puts more than a physical dent in the little ones, and a huge guilt trip on the person who was operating a plane that hurts a kid, regardless of whose fault it really was.