I do think it is good idea to go see Sumo, it is Japanese experience. What is the point of doing something at Tokyo which you can experience same back at your home?

I do agree with other that question will be how long can your 12 and 10 years old can keep themself entertained watching Sumo. My suggestions will be same as other that do not plan to be at Sumo for entire length of the day.

Usual Sumo daily schedule is that the stadium opens at 8:00 a.m. Sumo match starts from 8:35 a.m. Matches starts from lower ranked wrestlers and make way up to the top wrestlers at the end of the day, which end at 6:00 p.m.

Hardly any people go watch matches by lower ranked wrestlers, so matches in the morning are pretty empty, hardly any audience. Second half of afternoon is where places really get filled, I recommend during that time. My recommendation will be arrive around 3:00 – 4:00 p.m. and stay till the end at 6:00 p.m. Kokugikan where Tokyo Sumo event take place has small display of Sumo history and shops and restaurants, possible that you can arrive earlier and your 12 and 10 years old will enjoy.

Around 3:50 – 4:00 p.m. Yokozuna, current top ranked Sumo wrestler, will enter the center ring wearing fancy Mawashi (those Japanese fundoshi Sumo wrestlers wear) which signify matches by top Sumo wrestlers. The place really gets filled for this event. Very end right before 6:00 p.m. is Yumitorishiki, where a wrestler with a bow will be on the center ring, the ceremonial show by a Sumo wrestler which signifies the end of daily match.

Morning event is not a practice, it is actual official matches by lower ranked Sumo wrestlers which goes into official record. Since it is lower ranked wrestlers, usually there are hardly any audience during morning match.