I you leave Wi-Fi off, lockups will be virtually non-existent. If you do use it, disable it when you're done, and then power-cycle the phone. As long as you are in an area with good Sprint signal, there should be enough speed on the network for her use.
I run stock 2.1 on my phone. No ROMs, no root, no task killers, no nothing. Performance is very sporadic. I run some applications in the background (it is a smartphone, after all...something is always running), but not an extreme number. At times, the phone runs fairly smoothly, the battery charges properly, and all is well. At other times, with the same apps running, the phone is very sluggish, and the battery won't charge. I have not been able to isolate any cause for the phone's behavior. I don't leave WiFi on, I don't use Bluetooth, and GPS on or off makes no difference.
I don't know if your daughter has ever owned (or rather, you owned, and your daughter used

) a smartphone. You'll definitely need a car charger for her. I'd also suggest that she keep the supplied USB cable close by, so it can be charged off the nearest PC. The battery drain on this thing is like driving a truck that gets 10 miles per gallon.
There's probably a pink cover out there that will fit the Moment.