Get a bain-marie-type pan - a pan that holds water with a jug that sits on top - because that's by far the safest way of melting wax, especially for a 12-year old - or you could just get the jug, if you have a pan that would fit it.
Then I'd have a look on Ebay for supplies. For a starter set, I'd get a few plastic moulds, some dyes, some wick of the right size for the moulds, and you will need some blue-tack-type stuff for stopping up the hole where the wick comes out. You will need some wick pins, too - and of course, some wax. Just get basic parafin wax.
You're supposed to have a thermometer, but to be honest, the melting point of wax is so low that it's not strictly necessary - unless you go crazy with it and overheat it a lot, it's unlikely to catch fire. Conventional wisdom is that you need it in order to know when the wax gets to the perfect temp. for pouring ... but in reality, the perfect temp for pouring is slightly after the final bits of wax have melted.
It's one of those hobbies that a lot of people take up and then put down again, so you can often find unwanted stuff on Ebay. I'd have a look there before you buy all new.
Making floating candles would probably appeal to your DD - they look pretty in the water, there are some very pretty moulds around, and the other great advantage of them is that they're small, so they don't use lots of expensive wax.
If your DD gets into it, you can save loads of money by recycling your candles, and you could get friends and family to save their candle ends for her. I actually never have to buy new wax, because I seem to have enough with all the ends of dinner-candles I save.
Make her put some newspaper or something down on the kitchen floor OP - wax gets all over!