Ant – first – at that age I would sign them up as Rookies, not Novices. Better chance of racing in class believe it or not (or at least their age), and also better chance they won’t get combined.
Make sure they understand that racing each other is good, but explain to them the rules and track etiquette that ALL racers must abide by, and if they don’t, they will not be allowed to race. I would recommend you introduce them to one of the older riders, or better yet one of the officials and let them explain it to them as well, this way you are not the bad guy if they get thrown off the track for trying to take each other out.
Hopefully they will understand they are not the only riders on the track and they have to race safely.