I would use the rationale you mentioned.

I also explain to clients that knowing in advance of clients holidays allows me to know when I am going to be 'low' and than I plan my program accordingly as there are some field trips we can only do when I am low in numbers as well as planning for groceries and meals and so forth ~ for me it is not a 'requirement' with a certain time frame but worded that 'as much notice as possible of upcoming absences from the program is appreciated because of ABC'.

Also admittedly I need to know when they are going to be absent for a whole week or more so I can be assured to remind them if they are going to need post dated cheques over the course of their vacation to be left on hand .... cause in centre care we use to always have clients 'forget' and therefore their account was left with fees owing and administration wondering 'are they returning or bolting' and to start filling the spot and so forth and well while that is one thing to deal with in centre care cause it did not affect staffs direct income cause centres have large fees and buffers in the bank as a result .... it is not cool HERE where them forgetting to pay me directly affects my ability to pay my bills and so forth!