My son is 11, 12 in August. He's doing Honda Cadets at Rye House for this year. We are looking to move to Junior Rotax for next year. Was considering Mini/Micro (can't remember exactly which one) Max but as it stands there isn't a grid at Rye. This year is very much learning how to drive/race, next year will be all about learning to drive Rotax. We got his Project One last year when he was 10.
As a dad, with my son right on the cusp of a class change, I would probably look towards the Rotax route. If you go Honda Cadet you'll be replacing it within 6 months. Failing that, do arrive and drive (Club 100 I think it is at Bayford)
One thing I have noticed is that the Honda Cadets are VERY competitive, regardless of the track.
At Bayford, check out Ambition Motorsport, Paul has been very helpful for us when we visited and the Project One shop were as well.