I like Les Gets but I think there is more to do in Morzine – the town centre has lots of activities and games every summer and the pool is great with a big shallow bit for little kids. There is also a great lake just round the corner in Montriond.
Lots of walking by the river which is nice to dip in too.
You can jump on a lift with a bike and do a few laps in an hour or two and your partner and child can go up as foot passengers to meet you at a bar or cafe too. Easy access on lifts up to Avoriaz and over into Switzerland as well.
You can then drive down to Lac Leman in an hour or so – Thonon Les Bains is really nice to explore. Last year we also drove over to Annecy for the day – which was a bit hectic but ok.