Scotland is a great place to spend Christmas but then I am biased - I live up here all year round! But seriously, for the first time in years I spent Christmas and New Year in Liverpool with my parents because my Dad was advised not to travel after recent surgery and it all felt a bit flat compared to what we are used to and are determined that this year we'll go back to our previous practice of them coming up to me. Christmas was never the big holiday in Scotland - it's not that long ago that it wasn't even a public holiday - and I feel that it's still less of a consumer fest up here.
Aviemore would certainly be a great choice with plenty to do regardless of the weather and I can certainly recommend the Lodges at Coylumbridge - we've stayed there for 3-generation family holidays and everybody has had a great time - and would always prefer them over staying in the actual hotel. I've never stayed in the McDonald resort but others speak highly of it. You won't be guatateed snow but it will be cold so do your research carefully. A number of caravan sites stay open all year round to cater for skiers but you ahve to be hardy and dedicated to stay in one of these so-called 'lodges'! And it will be expensive - hence why skiers on a budget will put up with staying in a cold caravan in February - so much so that you could possibly find it cheaper to go abroad and with the added bonus of the greater probability of snow.
So, yes, Scotland would be a great choice but I would suggest that you also think about somewhere like the Borders or Dumfries and Galloway - a much easier journey assuming you live in England. Believe me - you don't want to get stuck on the A9 waiting for the snow gates to be re-opened at this time of year. Also, you'll find that prices will be lower in the Borders and that a day out in Edinburgh will also be feasible and great fun. There's a Continental Christmas market held in Princes Gardens throughout December with a giant ferris wheel and an open ait ice rink there too. Edinburgh has a really wonderful atmosphere over the holidays. But you will probably also find it relatively easy to find things for your 14 your old to do, especially if they are into being really active - there's great mountainbiking facilities in the Borders on forest trails for example. Alternatively, think about something within striking distance of Aberdeen if you want to go further north.
The thing to remember about Aviemore is that there are lots of things to do and see in the area but if the weather is too bad for the predominantly outdoorsy things, then your nearest cinema will be in Inverness, as will the nearest decent shopping - a big drawback in my experience if your 14 yr old is female with Christmas money burning a hole in her pocket!
SM