Hi,
I took my Australian nieces to Aviemore two winters ago when they were over for Christmas and we did the whole reindeer thing etc... and there was a brilliant slope just past Loch Morlich to sledge on. It is literally a 1 or 2 minute drive past the Reindeer centre, and is before you start gaining any proper height on the road to the ski slopes.
If there is snow when you go, there will be people sledging where I have described as it is a popular little area. There is a car park right next to the wee slope too.
Enjoy !