At my club, kids can start group lessons at 4 1/2. I think that is a good age. Your daughters will be fine, and if they have already shown an interest, all the better.
My club uses the United States Figure Skating "Skate with US" basic skills program. It's a great, well structured program, you could check it out on their website. They would start in the Snowplow Sam level.
Take them to a public skating session first just to see how they do. I've already tried taking my 20 month old out. But I'm a crazy skating coach, that's what we do to our kids!
I've taught little kids for about 8 years now. Let me know if you have any questions!
It's a really fun sport, but ditto that it's expensive if they want to start competing!