Dare2B is surprisingly good for kids' ski wear. Decathlon too. Trespass.
- how many pairs of ski socks
I'd go with 2 each, to allow 1 pair to dry if they get wet from snow. I'm afraid we don't wash ours whilst we're out there!
- would you hire a helmet? Would it be ridiculous to take along their scooter/bike helmets instead?
We hired helmets the first year but both DC caught nits from them (as far as we can tell). After that we bought our own, they weren't too expensive. I wouldn't use a helmet made for another sport because the protection needs are different, and I suspect most ski schools wouldn't take a child in a cycle helmet.
- do they need own goggles or can we hire?
Not sure I've ever seen goggles to hire, we bought ours, they don't tend to grow out of them that quickly so a pair will last quite a few seasons and can be handed down.
We take 2 thermal tops and 2 thermal bottoms each, again we don't usually bother washing them whilst out there, but they do sometimes come back wet from snow so need to dry out.
- they already have waterproof mittens - assuming these will be fine even for skiing one?
Probably. Assuming waterproof and padded.
- what are the best boots for them to wear to walk around?
Snow boots (Jojomamanbebe do a good range) or, once older, they take their walking boots.
Other stuff:
Salopettes, ski jacket, hat, fleece neck gaiter. 2 x fleece jumpers (regatta/millets do very cheap ones and they're absolutely fine).
We also take evening stuff, like trousers, warm jumper, normal socks. Sunglasses.
Snowsuit - depends on age, although warmer, harder to get on and off if need the loo during ski school.
I tend to dress the DC more warmly than adults, they tend to get colder and then get miserable quickly. Bright colours are excellent for spotting them on the slopes and when collecting from ski school. Some little hand warmers might be good to stick in mitts if very cold. Actual thermals not cotton leggings as warmer and will dry much quicker. Keep them fed and sugared up. Big breakfast, plenty of snacks, and keep sweets in your pockets to keep handing out when they start to lag.