Personally, I ask for 2 weeks as an 'enrollment' fee upon signing the contract which is used to cover the 2 week notice period upon termination of care.
I'd say if you're having trouble with parents just walking away without paying, then perhaps think of changing your policy, to have the 2 weeks payment you ask for being counted only towards the last two weeks of care before they leave, that way you're not getting left hanging with no payment for the last week(s) of care.