I've holidayed in Aviemore from since I was a wee kid, and love taking my wee ones there now to keep the circle going. It is a wonderful place for families, quite a bit to do in Aviemore village that kids will like (e.g. nice pool, go-karts), great outdoor stuff for them also.
Don't know the southwest well at all, but Peebles area (my wife is fae Roslin, you should go and see the chapel, well cool) certainly very nice and you have the double wonderfulness of Edinburgh and all it has to offer families close by, plus some fab scenery (although a bit tame and less intersting than highlands or my reco below)and awesome trail centres.
I would put a strong pitch in for Crief in Perthshire. You can get extrememly good value self catering accomodation at Crief Hydro that gives you access to all the nice hotel stuff, and it is a massively pleasant hotel that really understands families. Tons of activites in some of the most beautiful scenery in the UK. It is a quite a bit faster to get to than the highlands also. Oh yeah, and also within easy striking distance of Edinburgh should you wish.
When we move back to the UK, we're going to Crief. It's fab.