You can keep track a couple of ways.
Do you have or can you get a list of the kids ahead of time?
No list: Since they're not ordering anything at the time, all you need is a legal tablet and a pen. "NAME?" write it down. Shoot. Write down the ending frame (so if you shoot more than one you'll be fine). I find writing down the ending frame harder to screw up than writing down the first frame and less disruptive. An assistant would make this easier of course. If you do the preview bit and they pick one you can write than number in the next column.
List: Print out an alphabetized list (excel will do this easily) and then write the ending frame down next to their names. Missed kids will be obvious this way, no name mispellings, etc. Enter the data back into excel and you can sort on frame number if needed.
Best option is to contact H&H Color lab and get their EZ Event software - it's free. The barcode scanner is $200, the only cost. It's best if you have a list ahead of time, but can be done without a list. An assistant will make things flow MUCH easier.
You load into the software the list of kids - and teacher/class/team, whatever. It prints out the barcodes. Three pages basically - the packages (not applicable in this situation), the class/team/group and the list of kids.
Kid arrives - scan package, class/team, then kid. If you print the kids on labels you then peel and stick that label on their order form. Shoot kid.
At the end of the day you load the scanner data and CF card into the computer. It time syncs the scan to the image. It's bloody accurate - 100% with a practice session - errors are gonna be human ones.
You can then choose the poses in the software. It will assemble the packages and team/class photos automaticall. Upload to H&H (excellent prices on this class of product) and they will bag and label each student by class and sent it back to you for delivery. You no nothing else. Slick and cheap.