I am also a fan but can see that it would be pretty boring for kids.

If I might make a suggestion, why not just watch the matches on tv in your hotel room once or twice while you are there. Most decent hotels have access to the bilingual channel so the commentary and explanations are in English and you would have a MUCH better view of the action. The broadcast is always on NHK between 4:00 and 6:00 pm. One of the more interesting times is right at 4 pm when they do the colorful dohyo iri entrance ceremony by all the rikishi (wrestlers) followed by the yokozuna's (grand champion) ring entering ceremony and purification ritual. Those only last a few minutes.

Each bout after that will take 3 to 5 minutes with only a few seconds being actual sumo action. The rest of the time is ritualized preparation and stare-down.

If you go, you will most likely be in bleacher seating unless you want to spend a lot for a box. Even if you do that, you will not be very close to the ring as the ringside seats are pretty much all season ticket holders. Be sure to buy snacks and drinks on the way in. Everybody does that. You can also pick up a rental portable radio tuned to the English commentary simulcast.

I find the general seating at the Kokugikan to be very uncomfortable so even as a fan, I usually only attend in person one or two days to get my photos and the current bento meal while watching a few matches in each of the various divisions and then see the rest of the basho on tv.