For all who think about Mycroft and Sherlock as childs I just discovered this fan fiction:

http://sherlockcares.com/the-holmes-boys-series-3-the-brains-of-a-scientist-or-a-philosopher/

Tought it was really cute and searched for the beginning.

Series #1:
http://sherlockcares.com/the-holmes-boys-series-1-epiphany/

I wonder if it's as nice as #3; didn't read more of it, yet.

If you scroll down a tiny bit you find the Search field on the right side. Just have to enter Holmes Boys.