I think I can say that I’m a Japanese/English bilingual, but I also attended 2 months of French immersion while I was living in Canada.
One thing I can say from this experience is that mastering more than 2 languages that include Chinese characters and alphabets is a totally different story from mastering several alphabet based languages.
For alphabet language speakers, learning Chinese characters is a big hurdle.
This is not surprising because many young children in Asia, are driven to tears when learning Chinese characters (My daughter is no exception). Japanese children learn Hiragana, and Katakana, for which there are 48 characters each, as well as Kanji(Chinese characters) on top of this.
Therefore, young Japanese children take a lot of time just to practice writing. Children can do it, because its mandatory at school, but if circumstances otherwise, it would be very hard to keep their motivation up.
I respect English speakers who have mastered Japanese later on in their lives. It shows their intelligence and patience and adaptability to other cultures.
For children who have Japanese mothers and live in English speaking countries, their biggest challenge to mastery of Japanese is writing.
Without knowing Chinese characters, they struggle to read, causing their Japanese level stops at “kid Japanese” which is inadequate for working in Japan. (Although there are some exceptions.)
Shimajiro (http://www2.shimajiro.co.jpis) is a cartoon character that is super popular with kindergarten kids in Japan. With Shimajiro, it is easy to pick up some Japanese writing skill even though kids live in English speaking countries. There is a line of so called "Edutoys" that are promoted by Shimajiro, which are very well thought out and engaging for kids. Theses materials can be sent overseas as well. My daughter picked up Hiragana pretty fast with Shimajiro when she was only 3. I think using media like Shimajiro to catch children attention can help them learn Japanese up till about second grade.
Another thing I noticed about the alphabets is that the characters are mostly symmetrical, but Hiragana is not. The first letter あ by itself is fairly challenging character for little kids to write correctly.
Other than Chinese characters, another thing that makes learning Japanese more difficult is the grammatical difference from western languages. In English, we can say, " I want it." In Japanese, we express it like, "It want" and no subject. So we grammatically express things in completely opposite way. It took some time to understand the grammatical difference when I was young.
There are other points I noticed while teaching English and Japanese to my daughter and would like to share some of more of these in this blog.