Kids that young really don't need a full smartphone. I'd recommend a QWERTY keyboard phone for texting (since that's what they're going to be doing mostly), and that has Facebook / MySpace apps, and a screen big enough for some basic internet viewing (checking movie times, business info, the "reference" type stuff), they really don't do a whole lot of internet usage. If you're looking at Sprint phones specifically, you'd do good with one along the lines of the Samsung Seek, LG Remarq, LG Rumor Touch, Samsung Restore, etc. Landscape (sideways) keyboards are more comfortable for lots of texting than portrait (up and down) keyboards.
You can view phone calls made and received (phone numbers only, no names), the amount of text messages sent (but not the dates / times, people they were sent to, or the context of the message), and the amount of data used (again, no dates / times or what was viewed) on bills. The only way to get additional text / data information is through a subpoena for court cases. If you don't trust your kid to use it responsibly (well, relatively speaking, they *ARE* still kids and bound to be silly at times), DON'T GET IT FOR THEM!! If you feel they really REALLY *REALLY* need a phone to be able to get in touch with you at any time, get a prepaid phone, and knock the snot out of them if they use it when they aren't supposed to. I didn't get my first cell phone until I was 16, and that was only because I was driving out to the beach and back late at night for my job, and I had a POS car that mom was worried may break down at any time (never did, but the phone came in handy to let her know we were stopping for food afterward, so she didn't panic and send the cops out if I was late), and she (rightly so) didn't feel it would be safe for a 16 year old female to be wandering around at midnight looking for help.