Watanabe Kumiko is JunJun(heh, I'm redundant here) and has done many other anime roles such as Shippou in Inuyasha, Himi Tomoki in Digimon Frontier, Yamano Jun in Samurai Troopers, Munbei in Zoids and countless others.

Her voice is generally low and well great for JunJun! I felt she brought out her personality. 

Here is what she has to say for being a seiyuu when she was the guest on the seiyuu talk corner called Say You Club back in 3/7/97. Some of Kumiko's comments were: 

The reason why she became a seiyuu: 

When she was 15 years old, Kumiko came to Tokyo for the first time. There was an audition at the Sunshine building, so she just joined thinking that it would be a good memory of her trip to Tokyo. She wasn't actually thinking to be a seiyuu. But she was chosen as one of the 30 finalists. So then she started thinking that she might be able to become a seiyuu.

*English voice was provided by Mary Long.  She also did both Molly and Catzi.  I liked her voice, I felt it really matched well with the tough, and tomboy nature of JunJun.  Mary Long is also the voice of Hello Kitty's twin sister Mimmy.

Found this information on wikipedia as I have never seen the musicals that feature JunJun yet.  Wikipedia states that she is played by Hitomi Tomashino and Niki Ajima.  It also says the Amazones Quartteto were children around Chibiusa's age that were created by Professor Tomoe.  They can turn into senshi because they are part of Chibiusa's starseed.  I really want to see these musicals, it sounds interesting!