The organiser may have his reasons for this age limit TT but as far as I understand it the FRA gudance is that for races of 4 miles or less,ages 15 and above can run (and be insured accordingly) I may be wrong though!!

I organise a couple of short races and have had parents of chldren under the age limit ask me if their kids can run and have come up with a similar arrangement as you menton above.