But in English, a "Doo-doo" is one of a kids favourite pass times before they are capable of doing much other than drinking milk and sleeping. If Google "doo doo", and you get a lot of pictures of excrement (NSFW, probably).

Childish is an ironic way to describe it. :) It is a term that only small children use, but almost all American children at least know what it means even if they don't say it all the time.

I just realized that it's strange that there would be a slang term only used by children, but that remained in use by generation after generation of children over a period of at a minimum 50 years but probably much longer.

I'm not a parent, but I suspect that it's a word used by American parents during potty training, hence its longevity.