As far as I know there are several schools other the Cairngorm's own offering lessons still - I'd recommend School of Snowsports based at the Hilton on the road up towards Cairngorm (http://www.schoolofsnowsports.co.uk/) - disclaimer: I know some of the staff there fairly well, so am biased
I know they do kids lessons but not sure what age they'll take them from.
Other than Cairngorm, I'd also suggest the Lecht once they get some snow.