Addendum to contract

Tenancy Deposits

I have just signed and exchanged a joint and several contract with 5 students at a distance, and have received a cheque from one them for the deposit, which I have submitted to the DPS. (The other 4 are current tenants staying on for a second year and paid me cash last year). The cheque was from the student's parent, a fact which had not been discussed earlier in the negotiations. Consequently, I do not have the parent's name on the new contract as a third party. Can I write an addendum to the contract to add the parents's name, and what form would this take?
Failing this, what can I do to cover myself?
Thank you in anticipation.