Ok i will give this a shot, i think i got this whole thing down now
Morning musume from what i understand is the most popular female singing group in japan, i am not sure about their music being specifically for children but if it is maybe someone can enlighten me to that fact. The group is composed of 12 girls from ages 13-22 i believe and the group frequently changes members (ok not that frequently but periodically).
The last time i checked Ayumi Hamasaki was the most popular female solo artist in japan, but she really doesnt have anything to do with morning musume but she did sing with mini moni on some show that i saw on google video.
Morning musume is also part of something called the Hello Project! sometimes referred to as H!P as an acronym. The hello project is run by this guy with some name that starts with a T. There are other groups that belong to the H!P such as H!P all stars, Mini Moni and several others. Mini Moni is a sub-group of morning musume formed form the members of morning musume that are less than 150cm tall. and i can assure you that mini moni songs are specifically for kids even though i believe the members of mini moni were actually a bit older than they sound. You dont really have to worry about mini moni now cause they arent really a band anymore.
I wonder if they really have bathrooms that are that big in japan, i would doubt it though....
I am probably way too old to listen to morning musume but i do find their music to be quite good, you can laugh at me if you want.
I have heard the song for Tommy February 6 everyday at the bus stop just havent seen the video for it yet, i dont particularly care for her music. I should probably check that video out for a good laugh if i can find it.
Shingo Mama no Gakuen Tengoku as far as this song goes i havent heard it yet or maybe i heard it but i am just not recognizing the title hehe.
This is the best i can do for an explanation of the whole thing from my head, i am sorry but i cannot explain why they are in a mens bathroom or like, any of the other stuff you mentioned.
Ok, i am off to go try to find some of the videos mentioned by Kirin Lemon cause perhaps i am missing out on something here, heh.
Oh yeah, ok for the person who posted the Yatta Video, that is an actual song sung by an actual band, i think the name of the band that sings that is called Happatai (or something close to that spelling!). Thanks now every time i hear that song on the radio i am gonna think of that video....eeek!