I would recommend Val d’Isere rather than Tignes.
Firstly, the village is high at 1850m. Not quite as high as Tignes at 2100m but close. Both adult and child are beginner skiers (and presuming it’s both thier first times skiing?) they aren't going to progress off the nursery slopes for at least the first 2 days. Val d’iseres nursery slopes are quite big and right in the middle of the village - easy to get to, off a pretty high st, lots of stuff around when you need a break. Once you progress a bit the instructors will take you up the Solaise lift to the Tranquil area which is high (2560m min) and really easy / chilled out skiing. You can get the lift back down again at the end when you are tired rather than having to ski.
There's also the option of staying / skiing at Le Fornet end of the resort. It’s linked to the main high street / resort area by a free bus that runs every 15 mins, all day. You get a gondola up and this side of the mountain is sunny, very quiet, with mostly big wide blues. Also high altitude. Excellent for progressing onto, although the beginner lessons will be on the nursery slopes in the middle of town (unless you have a private instructor).
The reason i’d recommend val over tignes is for the non skiier. It’s pretty, there are lots of shops and places to eat, you can wander around the old part of town etc. I’m not sure how much there is for a non skier to do in Tignes. Also although tignes ski area is amazing, it’s not so good for beginners (I used to take guests to Solaise and Le Fornet in Val for the views, the sun, and the quiet slopes. Tignes was usually busier).
Val thorens is a good shout (avoid meribel and the other villages in the three Valleys as they are low and there will be lack of snow).
personally wouldn’t recommend La plagne, it’s a faff to get around.