book full fare ticket for the child, RAC or confirmation will not be given to him on extra priority but if the ticket is WL and gets RAC/Confirm from WL, then only he can travel.
Otherwise the WL ticket will cancel automatically as full PNR will remain WL (if booked online)
if booked from counter, first of all, it will be hard to take counter-person into confidence that child can travel alone or his ticket is being booked now as he was left for booking from a group whose all tickets were booked...
more... earlier but this child was not planned to take earlier with group. if agree, he will book and in that case on WL also, he can travel.
But if really, he is going alone, who will allow him to sit with him if his ticket remains WL (not cancelled), even if booked from counter after befooling counterman.