Custom account payment

Looking for the best (easiest) way to set up an account payment system.

My client runs a daycare centre and would like her clients to be able to login to their account, view and pay for any outstanding fees.

The amounts will be manually entered as it varies depending on the parents income. She understnds this and is not a problem.

So bisically, I need to set up a system where she can select a client that has registered, enter an amount that that client owes and allow them to pay right then. Ability for partial payments would be great.

Any ideas anyone??