you can charge whatever you like, parents will decide whether your charges are unreasonable based on other CM's locally, how much they like you, how well off they are etc.
My Cm factors 4 weeks holiday into the fees and divides by 12 so I just pay the same amount every month. if she didn;t charge for holidays her rates would just increase slightly to result in the same overall income for her so IMHO its a red herring.
Parents know what they can afford to pay, they won;t pay more than that but they will pay up to that amount to get a CM they prefer. (At least I do). IME CM's who don;t charge for holidays do charge a higher rate per hour.