HI B- no one will guarantee snow - certainly not to base a holiday on and as some of the lower continental resorts - below 3000 m in Italy,France, etc are having lack of snow, be prepared to keep the little ones entertained with other things for the break.
Aviemore, at the Cairngorms, has other attractions around it like the Wildlife Park etc though the Nevis range / Anoch Mor, with Fort William as a base, is a higher resort.
I would suggest one option to look at for accommodation is trawl through the info at
http://www.visitscotland.com and look for Kingussie, Newtonmore or Aviemore for the Cairngorms, and Fort William / Lochaber area, for info on the b&b's etc. There is also some stuff on winter breaks which may be worth checking.
As above, January is more likely to be colder / chance os snow. Be prepared for winter driving experience though - slow roads due to ice, road closures if there really is a blizzard, and dark early nights - all part of the fu though and don't be put off - the scenery is spectacular and a clear day full of bright sun, ice over the lochs and snow on th ehills is fantastic...
Hope you have a great time!