Your other option to address the 10 vice 8 days is to charge for 8 days as in the same as the other family at 2 days per week guaranteed whether they come for two or not - that just holds the space for them. Any day over that they pay an additional fee. In theory if in a two week period they came only one day one week they would still pay for two and if the next week they came for 4 days they would pay the two day rate plus for two additional days. What you charge for the additional days will be up to you - as in as close to another daily rate as you can go without the family bailing on you. At least no matter what the family does attendance wise you know your basic income for the week that you can budget with.