When a player is inbounding the ball how far should the defencive player be from the line this is for 1 to 12 year old players.

You'll need to check with the league and/or tournament you are in. However in my area, most leagues and tournaments follow the state high school rule book with a few modifications which they send to the coaches. In the state of Iowa, the rule says...

"The opponent(s) of the thrower shall not have any part of his/her person through the inbounds side of the throw-in boundary plane until the ball has been released on a throw-in pass."

To add to Jeff's point, at the younger levels and especially in funky gym situations, refs will back up the defender to allow room for an inbound pass. We played in all kinds of gyms that had bleachers pulled down on the sidelines or the space under the goals was tighter than normal. At the 5-12 age group rec level, I think you'll find the refs making a judgement call if they deem the inbounder needs more room.