Sounds like you are pretty set on doing it your way and the way you have it in your contract. There is nothing wrong with that BUT you do NEED to tell the parents this ASAP. Otherwise both parties will be frustrated.
Some kids do magically get it at age 2 and in a matter of days they no longer need the constant reminders. It is totally up to you to try it or not.
I would suggest you decide now what you are willing to do and tell them in person and in writing. Explain your reasons and what you will and will not do. They can take it or leave it.