we were chatting once about itouch apps, and you said you thoguht your ds would get more out of it if youtube was turned off, but you didn't want to delete it as he gets lots of use from it?
I have just found out how to disable it quickly... (you probably found out months ago 
)
anyway - go to:
Settings
General
Restrictions (if none already set will ask you to set a passcode)
then you can toggle you tube on/off.
when off, no icon shows.
sorry if you've been doing this for months...
(there is also a handy "in-app purchases" toggle, which you probably want to toggle off if letting any child run free with an itouch)