Sounds like they do not want to pay for their vacation time, but they should have told you this in advance and wrote out a new cheque for whatever you agree on. Yes, try to email or text them and ask what the heck is going on? They can send money to you by email or Western Union if they have to. I feel this was sneaky as they did not mention a word to you about payment for the month of July. See if you can get ahold of them first, then advertise their spots. Is your policy to charge Flat Rate (pay whether kids are there or not). If so, then yes they are skipping out on you or just being cheap!