We were there 3rd week of Jan and used Oxygene for the 10th year running; they have never let us down and always seem to have a good ski school for children. One year I suffered an injury (broke my leg) and they really looked after me. So we keep going back to them. That said I have never heard any complaints about any of the other schools! For fast lunch and good value for money with kids try Croq' Neige it is next to Oxygene ski in Plagne Centre.
One bit of advice for all, when you purchase ski passes also buy the mountain rescue, it is very cheap as insurance and if you have it you are unlikely to need it! I did not when I hurt my self and it was expensive. The rescue teams are fantastic and had me off the mountain within 20 mins and at the Medical centre. Enjoy you skiing. Chris Marshall.