Rod,
For the Comers I can tell you 66 is the best for all three tracks here in Colorado, but with a big ?IF.? This gearing will get your kid around the track the fastest, but only if, he or she keeps their foot in the gas (WOT-wide open throttle). If they are not quite there yet and let off in the corners then it has been my experience that this has worked best for us: 66 to 69 at IMI and 69 to 72 at Bandimere. Again every kart varies to some degree but the key with the Comers is keeping the momentum up and when they can do that, get them to the 66 tooth sprocket. One more caveat, this 66 tooth works really well for rolling starts, but if they do standing starts, obviously this gearing will cause the slowest standing still starts and thus your little one will need to be proficient in passing slower karts or go with larger numbered rear sprockets.
Hope this helps,
Don