Have not been to a sumo tournament, but if I had a 10 yr old and a 12 yr old, Sumo would be one of the last things I would do. (I have a 12 yr old son, and visited Japan with him when he was 10 yrs old)
Alternatives I would suggest
Disney land/sea
Miraikan (museum where Obama visited)
Tokyo Tower/Skytree
Harajuku/Shinjuku
Kamakura
Train museum
Ueno Zoo
Museum of Science (Imperial Palace)
Museum of Natural History (Ueno)
Various aquariums
Kamakura
Enoshima
other things in Odaiba
Tsukuba mountains
Hakone Mountains
Izu
......etc