Damage deposits

Tenancy Deposits

My tenant is on a rolling contract, having been with me for years. We didn%E2%80%99t take a deposit when she moved in. (I would now!) her circumstances have changed and she%E2%80%99s now looking at fostering kids - specifically troubled teenagers. She%E2%80%99s asked if we%E2%80%99d be ok with this as we would probably be contacted about it. My question is would we legally be able to now ask for a damage deposit, specifically to deal with any damage caused by these kids? Thank you