Here's the RFU regs -- other jusrisdictions may differ
[LAWS]15.14 Duration of Matches, coaching and training sessions
15.14.1 All players, match officials and clubs must ensure that no player plays more
than 35 matches per Season and in respect of all matches and festivals,
plays no more than the maximum playing times set out in the table below:
Age Grade Maximum minutes each half Maximum minutes per day
U7/U8 10 50
U9/U10 15 60
U11/U12 20 70
U13/U14 25 80
U15 30 90
U16+ 35 90
Further guidance as playing times and durations for single fixtures,
festivals, tournaments and coaching and training sessions are set out in the
Guidance available at
www.englandrugby.com/Regulation15Guidance.
15.14.2 No extra time is permitted in any match except that added for injury time[/LAWS]
It's clear that there is no extra time - but I'd say that as long as the kicking competion fits into the maximium minutes per day - then you are OK.
(but in age group I wouldn't have a kicking competition. I'd settle it on tries scored, first try scored etc etc and if all else fails a coin toss, I reckon for an child a coin toss is better than a kicking compettion)