The under 13 age group uses the full size pitch. The coach should consider that the 5m tram line as a bigger obstacle for a 12 year old thrower, than it is for a 15 year old thrower
To develop lineout understanding, it may be worth exploring for the best thrower and not select their roles at the lineout by shirt numbers

If the policy is to throw to the front at every lineout, how will this develop the tactical and technical understanding of the lineout game?

It may be that possession is occasionally lost, but is the long term development of players more important than any given children’s’ game?