The TracFone might do the trick. I have one that I use mainly for texting my BF through the day with little odds and ends that wouldn't be worth a phone call. As long as it's used mostly for texting or for phone emergencies, it's pretty economical. And it doesn't offer any games that kids would find interesting for more than about 2 minutes, LOL!....If you get her something like a TracFone, she could send you a quick text message (e.g. "On my way") just as she puts foot to pedal. That way you'll know she's in transit.

I can't remember if you mentioned adding a horn or other noise-maker to the bike. If not, it might be worth a few extra bucks.

Also, if she likes lights, think about adding Tireflies to her valve stems. (I've seen them at Target and WalMart.) Or there's a gizmo I saw recently at Target made by Schwinn that attaches to spokes---forms designs or words as the tire spins---easy to turn on and off. Would add another level of safety if she's riding on a gloomy or dark day..........The Road ID folks also sell Supernova Fireflies in green, blue and red that can be clipped on to a saddle bag or worn on a wrist band. Turn on and off with a simple twist. Very bright. To my BF's amusement, I have one in every color! Check them out at www.roadid.com.