I agree with what you want to do, I've recently done the same. I think more than 30 days is more than enough time for them to decide if they wish to stay with you. If they don't, oh well. I would also give them until mid Jan to let you know if they will take or leave the spot, that way you have time to fill the spot if needed.
If you don't already, I would add in how much time you need as notice to take the child out of care.