When I've travelled with families everything is paid for. All flights, food, accommodation, car and fuel when there, ski passes, equipment such as skis, thermals and ski wear, travel insurance, phone or phone bill if expensive, and expenses such as taxis. Anything I do out of work I would pay for such as sightseeing, meals out or shopping. But they would make sure there is enough food in the fridge for all my off duty meals too. Normally I would just add what I want to the shopping.
If travelling on a day I don't usually work I get paid a full days pay and an hourly rate for any extra hours worked. This is even if not travelling with the family, if they have travelled separately for example.
It's not a holiday, although it's normally quite fun. My families have always treated me like part of the family and have asked if I want to go out with them on my days off, I normally politely decline but it's nice of them. And they always try to keep the kids out of my way on my days off so I can have a bit of a rest 
Babysitting is optional on my part and charged at normal rate per hour. I would make sure you chat about this before you go if you want lots of extra hours just so you both know where you stand, I used to work long hours anyway in these jobs and didn't really want to work loooads more hours.