Teaching them is pretty easy. My son started just before he turned 3, and my daughter just after she turned 3. With my son it was a little early on hindsight - We lived in Colorado then, and I remember him falling asleep in the afternoon on the lift up and having to ski carrying him down the slope.
For me, it was really easy to teach them by using a harness - there's a couple of types, but it's basically just something that goes around their waist and has "reins" going back so that you can control the speed and turn them. (Use the reins like on a wagon - for my son he was a horse and he was pulling me down, and for my daughter, she was a reindeer, and I was Santa.) We used the "Pizza Wedge" expression to teach them to snowplow. And Santa can't use poles for a couple of years - to hard to mess with the reins and lift them on and off the lifts.
But my personal opinion is that's only good for a couple of years. At about 5, get them some proper lessons from a proper ski instructor. In one 50 minute lesson, my daughter went from a snowplough to almost parallel turns. If you have the ability to start with a real instructor, that's probably the way to go, but for us, it wasn't an option because of the timing of the classes.
And they know no fear at this age. The problem is that they just want to ski straight down the hill, and will do almost anything. My daughter wanted to ski the whole resort last week (it's a small one), and she did, including one red bump run with a lot of loose snow - nothing groomed there. I was more nervous than she was.
But like has been mentioned, just keep it fun - you don't have to push them - if you do it will be a disaster, and no fun for anyone. If they figure out something is hard for you, they will really enjoy wearing you out.
Oh, and try to buy used - one meter skis aren't that much cheaper than 177's and new skis and boots every year gets expensive after a while.
Smallest twin tips I've found so far that she likes are 130s, but I think I've seen 110s. Since they're probably not landing switch at this age, true twin tips aren't really necessary - for my daughter the amount of pink is the most important thing for next year - and preferably with princesses :-).