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To hold a space I would charge them a minimum of three days a week so you know how much money you are going to earn. If mom then works more than three days she would add a daily amount accordingly.

The other option since there are two children would be to charge one full time fee so getting paid for 5 days per week and then if more days are used (remember to count two days for each day they come ie one day per child to get your total). Again it allows you to know the minimum amount of money you will earn.