What oil to use? In what ratio? The C50 uses 2 stroke oil, and Castor is faster, but leads to carbon build up more than synthetic oils, so keep checking your head volume (see separate post). How much oil to use? The Comer C50 manual says 40ml of oil per L of fuel, engine builders say 50-100ml per litre of fuel.
The best way to find out what works for you is to get your child to a point where they can put in consistent lap times (within a few tenths when in free air). Use 60-70 Ml of oil, or what your engine builder recommends.
Then change the oil amount by 5ml, keeping the same jet and see how their lap times are, keep a record of the oil mix and the times and see where you go best. If you change jet and oil together you won’t know what made the difference, or you may even see no difference, although you will need a bigger jet for more oil, so test different ratios on different jets.
Keep a spare Jerry can for fuel that’s changed, you can always use it for practice or a two stroke generator if the mix doesn’t matter too much.