Hakuhinkan ginza shop, vstone robot shop are good places to collect souvenirs for kids
Hakuhinkan Ginza - http://www.hakuhinkan.co.jp/guide_en.html is a place to visit for sure. Here you can find lots of toys, puzzles, gadgets from Japan that will surely be liked by kids from any country. Its an awesome place.
Vstone - kids robots building place - http://www.vstone.co.jp/english/index.html - here you can find educational robots - so techie kids will like it very much.
Then offcourse if you visit places such as Asakusa shrine or even in Hakuhinkan you will find masks of power rangers, samurai, plastic swords etc which can be every much fun for kids.
If it is summer, yukata or kimono for kids is also a good souvenir.