The Three Valleys is good - this is where we usually end up though! We went to La Plagne last year which was also good and lots of ski to the door chalets but I did think the pistes were more crowded there.
If you want to fly from Cardiff or Bristol and have a package holiday with in house childcare - Crystal is the way to go imo although you would be looking at a club hotel rather than a chalet. If you have two under two who might fit in a cot then this might be perfect for you with free child places, childcare on the ground floor and ski school just a minute's ski down the hill. Not the cheapest and not everyone on mumsnet likes crystal but we have been with them for the last two years and are going back to the clubhotel downstairs from this this year and think they are fine.
fly to chambery and the transfer is under 2 hours depending on who else they have to drop off - might be worth a private transfer with the little ones. There is a mumsnet horror story about a half term transfer to this exact place with crystal in 2008 here as well as my write up on how good a time we had.
La plagne is brilliant,it has got skiing for every ability and good quality equipment. It has some lovely restaurants and self catering chalets. We ( another family and i ) split the cost of a ticket and took a nanny with us...it was brilliant. She used to meet us at the top lift for lunch with the babes and they got the fresh air and the sunshine too... they slept soundly at night too!
snowbizz could do it for you - the catering would be in a restaurant in the appartment block (which you can book for a week). it is a small resort, but it fab, and you could probably arrange taxis to serre chevalier if you were so inclined.(it has also been very snowsure in the past)
the appartment block ski school and creche are in the same buidling at the bottom of the main lift. they fly from london and manchester, if that would fit with your travelling!