No time like the present. January is too long to wait, especially if you aren't really changing anything, simply enforcing things that you have let slide so far.
I agree that you should re-assess your paperwork, make any necessary changes and send home a new copy with each family. If need be, highlight any areas that you have concerns about so that you know that they will notice. Let them know that they have a certain length of time to read and review, then return a signed agreement. Should you not have it in your hands by a specific day, consider that notice and begin advertising.
Your business needs to be run in a way that works for you, not just is convenient for parents. How would they feel if their boss said "Nah, we're shutting down today, so no paycheque for you."?? I have begun explaining to parents that their fee pays for a "spot" in my daycare, to ensure that thier child has a safe and secure place to come WHEN THEY NEED IT. I have tried to get them to stop seeing it as "I've paid for those hours/days". (Although if you work it out, really they don't pay that much hourly, you know??)
Don't stress too much about it, we've all made concessions as we get started that we later regret. In the beginning, it's so important to fill those spaces that you forget that you are stuck in that situation indefinitely! If someone walks, then at least you know you are replacing them with a better scenario for you. Good luck.