Do it now!
Print off a copy of your contract for all your clients. Tell them that you have decided to use a contract to lay out clear expectations of both yourself and the parents. Tell them that this is a way to encourage good communication so that misunderstandings are at a minimum and you can focus on providing excellent care for their children.
Tell them that they have 2 weeks to consider your contract and return it signed. If they do not wish to sign the contract then those two weeks will be considered their 'two weeks notice' to find other care.
Make your business what you want it to be! One of the families might walk, but I'm sure they all won't when they realize that a contract protects them too. Spin to be a positive for them and they'll be glad to sign.