6 months still isn't a very long time and whether you act like you just found out or not, I think they'll be able to do the math and figure that you knew regardless. I understand the position you are in but if the roles were reversed and I was that parent, it would leave a bitter taste in my mouth. To only be with you 6 months and then have to look for another provider, I'd only want my child to have to transition once if I had any control over it which if you are up front with them, they would have that choice.

Take that one family out of the equation for a minute and I think it's perfectly acceptable to withhold this info from your current families if for no other reason than to get through your first trimester. While withholding this info for any kind of financial reason makes sense for your income which I do understand, I think the other family is an exception and regardless of your need for income, I personally think being upfront with them is in the best interest of this child and their family.