We went to Trier christmas market one year pre DS, it was lovely all the buildings and stalls and lights. Not too touristy but busy, with the best stalls being hidden down all the little streets as well as the ones in the centre of town. We bought nice xmas decorations mainly, and some traditional toys for neices and nephews.
Would love to take DS he would find it amazing. Can't afford it this year though.
I like Valkenburg, because its in caves and is truly beautiful.
If you don't mind small and sweet (or even chocolatey) then there is Bruges - if they are still doing the ice sculpture exhibitions that they did every year, this is a MUST see (although not a market, admittedly). Bruges is small enough to get round on foot and has pretty shops including oodles of chocolate shops
...what is the nicest, most traditional, Christmas market? I'm thinking of going to one of the German ones but would really appreciate some advice (have to give people loads of notice which is the reason I'm asking now.)