A few notable ones and favorites:
Carl Craig, who lives in Virginia now (I met him at Asian Fantasy Film Expo 2002, and he's a great guy!), is famous in his one starring role as the first gaijin kid in the Gamera film series, as boy scout Jim Morgan in Gamera Vs. Space Monster Viras (Destroy All Planets in the US).
Tomonori Yazaki played Ichirou Miki in Godzilla's Revenge, and was in the first Masked Rider series.
Hiroyuki Kawase starred in several Godzilla films (he played Ken Yano in Godzilla Vs. Hedorah, and Rokurou "Roku-san" Ibuki in Godzilla Vs. Megalon), and was in several shows like Mirrorman, Meteor Man Zone, and even Japan's own version of Sesame Street (Open Sesame) in the 70s!
Akinori Umezu had a brief cameo as one of the two city kids in Terror of Mechagodzilla (the two kids who were going to be stepped on by Titanosaurus before Godzilla intervened), and later played one of the five heroes in Toho's Saucer War Bankid.
Youji Matsuda, as a child actor, played Masahiko Okamura, the kid companion of Masked Rider Amazon. He also appeared in Ultraman Leo and Deathquake, before also becoming a noted voice actor, one of his notable roles was Ashitaka in the Japanese version of Princess Mononoke!