@choas – I am in Samoëns right now and it feels like summer. Although glorious sunshine it’s too warm for mid Feb.
Anyway I might be biased but Samoëns is almost better in summer. Absolutely loads to do with kids. I have two the same age as yours. For the riding the Samoëns official vtt trails are too tough for these ages so we stick to the valley floor with plenty of trails along the river. There are various shuttles that run up to Les Gets (15min) if you want a bike park. For yourself you can get the 2 or 3 main marked trails (rooty and natural) done in a morning using the Grand Massif Express telecabine. Plenty more unmarked but you probably want a guide (or at least a riding buddy).
Aside from the biking I would recommend fat scooters at Morillon for a half day. White water rafting and swimming in the various lakes are my kids favourite.
If the weather is good you won’t be disappointed.
Doz