A couple come to mind...
Johnny Borden (Bye Bye Birdie, Oliver!)
Freddy Michaels (Mame)
Andrea McArdle (Annie)
Daisy Eagan (The Secret Garden)
And I don't know who they are, but I love the little kids who fight while singing "If You Haven't Got a Sweetheart" from A Tree Grows in Brooklyn. ("You've got the words cockeyed!" "No, I haven't!")
"Gif me the cobra jool!"