The one time I visited Japan during a tournament, we went. We got the super cheap seats sold only on day-of, which are not seats I would recommend for anyone really into Sumo. They were the at the very top row of the stadium, and were regular sets (i.e. not box, not floor seating, regular stadium seats). But, they were so cheap that we felt no obligation to stay for any particular length of time, and we wouldn't be too bummed about it if we just found the whole experience lame.

We didn't find it lame, at all, though. If you go, you'll see something uniquely Japanese, and a particular slice of life you can't really experience anywhere else within Japan. It's possible that it would be boring for the kids, but on the other hand, all the pageantry associated with the top level guys is pretty crazy and they might like it.

I say give it a try with some cheap seats early during your time in Tokyo. Show up in time for the top classification group, watch the entrance ceremony, and one or two bouts. If you're having fun, stick around for the rest of the day, otherwise, go get an early dinner