I'd look at a basic but full size Janome or any other brand that has speed control on it. With the Janome you can slide the control from slow to fast and no matter how hard you press on the foot control, it won't go faster than the set rate.
I started sewing doll clothes when I was 8. i think if they're showing interest - tap it!
The problem with a kids machine is they will outgrow it, and they will be looking for it to do more, once they get the basics down.