Like the other guys I can't imagine that there is a certain score they should shoot. To start with there is a HUGE difference between the average 11 year old and the average 14 year old.
A lady asked me to play with her son in an adult/child tournament one time and they had the tees set up at about 3,600 yards for under 12 and under. Seemed about right.
The kids over 12 hit from the ladies tees at 5,650 yards.
A couple of the 14 year olds were driving most of the par 4 greens from those tees. Of course they were used to playing from the white tees at 6,254 yards.