_foo_bar=Something&baz=Else

could parse as either of:

  {foo:{bar:"Something",baz:"Else"}}
  {foo:{bar:"Something"}, baz:"Else"}

which seems to ruin things. You need a character to end an object, maybe ; or & would work.

Also, I don't like using : in query strings as a separator since that makes it a bit ugly and not quite a query string. How about using = for both and using say ! to identify non-strings, and @item@item@item for arrays?

So your example becomes:

  _user_name=Bob%20Smith&age=!47&sex=M&dob=5/12/1956&pastimes=@golf@opera@poker@rap&;children_bobby_age=!12&sex=M&;sally_age=!8&sex=F