I agree that a 99 with a hand crank is a good option. Kids (adults too) love sewing on hand cranks. The 99 also has the advantage of using cheap and common class 66 bobbins, low shank attachments, and 15x1 needles so there's no hunting for obsolete or proprietary feet, needles and bobbins.
I also recommend getting the attachments and a buttonholer as well.
Olivia recommends the 401 and Touch & Sews for the decorative stitches. My only issue with them is they're not as straight forward as other machines to operate. I do think a simple zig-zag machine with a few built in stitches is a good choice. Olivia likes sewing clothes. If you're sewing knits you want a machine that can at least zig-zag.
I don't think the brand or model is as important as features. I would recommend something that takes class 66 or 15 bobbins, 15x1 needles and low shank feet. They're all easy to get. If she wants to sew clothing then a zig-zag machine is good. Kids love decorative stitches too. If you DGD is going to 4-H meetings a lighter machine is a good choice. We used an old Montgomery Wards machine last year that weighed 40 lbs. A good machine but not the easiest thing to carry to and from the car.
Rodney