I recommend you call a local babysitting service, perhaps one that advertises through a hotel....ask them their policies and copy those with whatever modifications you deem appropriate.
I used to use one service in Phoenix, only about 2x year and they had a strict cancellation policy, which I don't recall anymore.
Anytime you hire yourself out for a fee for service type of job, you must have a protective cancellation policy...because you have made a commitment to be there and available and have had to turn other customers away.
So yes, start a cancellation policy immediately. And if you travel further than 10-15 minutes away, I'd start adding a travel fee too. But keep it reasonable, as I have turned certain businesses down. One electrician wanted to charge me $100 just to drive to my home in an area he normally services.
Also, I remember answering your post about the last minute voice lesson cancellation...so this seems to be a common theme for you....so even more important you are crystal clear, upfront what your rates are for everything....baby sitting, voice lessons, cancellations, driving far out, etc.