Cross Checking is illegal in most lacrosse play - girls, boys, all ages. For completeness sake, please note that box lacrosse has modified this rule and in some cases it's allowed.
The cross check rule (rule 5.2) is known to be one of the most simply worded in the book - “A player may not check his opponent with his crosse in a cross-check position. That is, a check with that part of the handle of the crosse that is between the player’s hands, either by thrusting away from the body or by holding it extended from the body.”
For outdoor lacrosse, this is what a cross check looks like.
This example is especially good as the crosse hits above the shoulders, which makes the foul clear to see.
Check out this great article about boys youth, cross checking and how it's viewed by officials through the US Lacrosse lens.