1st off you have to put YOUR mind back to being a kid. Why did we start off pumping those single speed BMX's all over town? It was fun. My 7 yo got her 1st bike with gears this year. 1 chainring and 7 speed hub. Since they know everything, they hate to be told what to do and constant reminders of what gear to be in just turns them off. Your best bet is let them figure it out. Get a slower bike (for you) as well. I use my cruiser or touring bike when I ride with the kids. Just slow down let them ride their pace and enjoy the ride. Stop for lunch or ice cream and enjoy the company of your son part way through the ride. And also realize that your kid may not care about riding, they just want to have fun and spend time with you. Whats the worst that can happen if they ride in the wrong gear? Get tired and need a nap or have to push their bike up a hill. Just get off and push with them.

As we get older (us roadies are the worst!) we forget what fun the ride can be if we just slow down and enjoy the scenery. I have routes that I ride with a group that I never really see until I go with my kids.

Oh, let them grind and clunk the crap out of the gears....so what, it's just a hub. Remember when you started driving or riding? A rear hub is a cheap price to pay for time spent with your kids.