My son's school (secondary) uses a thumb print system, but once they have their print taken it is converted into a digital number sort of thingy :) and the original print is then destroyed. They can't pay for food and drinks themselves but scan their thumb to purchase them electronically.
Our school uses Parent Pay which costs nothing for me to use, the school issues you with a password and log in ID which you can then change once you have logged on for the first time. I pay for everything connected to the school this way, you can use either a Debit or Credit card to pay and the site will now store your most used card so that you don't have to keep putting the card number in each time.
You can go into Parent Pay any time you like and top up your child's account. The school add all your child's purchases so you can keep track of exactly what they are eating and drinking for their break and lunch each day. I find it very simple and convenient to use.
Jan