I am advertising for a preschooler as well and I get tons of infants. The parents all tell me, "I know your looking for a preschooler but perhaps you'll reconsider for my special little guy". I've had a handful actually use wording similar to this! I want to respond saying as special as he is to you he's no more special then the other ten 12month olds asking for the spot!!

I can't believe they lied though!!!

I would just do an across the board $5 increase for November 1st then. Sounds like they can manage it.

Remember NOT to apologize. you are doing nothing wrong for increasing by apologizing you own up to it being your fault when it isn't. Just simply point out the reason, the hardship you now face and the cost increase. Leave it at that.