Good motor for young, total beginner?

[VintageRacer]

I've bought my son a Mad Monkey kit for his 9th birthday and am wondering what is likely to be the best motor for him to start racing.
I realise a slower motor is likely to be quicker round the track to start with, but how slow?
I was originallly going to stick with the supplied 23 turn brushed but I have a 17.5 brushless that I was intending to put in my Optima. I am now thinking this might be better as a atarting motor in the Mad Monkey.

Will he be too slow with either of these and should I be considering a 13.5 or even a 10.5 ?

Thanks.