I would do one of 2 things - either give them notice now that your rate is changing and give them the opportunity to look elsewhere (I would refund their deposit if they do go somewhere else as you aren't following the contract that they originally agreed to) or charge them the old rate and give them notice after a month that you are increasing your rate as of July 1st. I think if you change the rate too quickly you risk the chance that they leave anyways, once their child has been there a few months they will have more of a relationship with you and might stay despite the increase.