Brother CS6000i is a good cheap machine...do a search here for recommendations or read other reviews online. It's lightweight, not hard to use and has lots of features that would grow with the girls. You can check Walmart, Amazon, Overstock.com or other places to compare prices.
I would avoid the cheap new Singers, they are just frustrating. I'd pick a Brother before I did a Singer.
Don't overlook Sears Kenmore, which posters say are made by Janome and have many of the same features in a more affordable machine.