It will take them at least two weeks to set up interviews and start again to find someone if everyone they interviewed the first time is already filled. It can also mean they are on a wait list for a centre if they aren't concerned about bringing the child to you in Sept knowing that it will be only for 6 months.

Since the family never gave you any indication on where they were at when you contacted them you also have the option to say you know I have thought about it and given how am feeling being pregnant I have decided that I am not able to take such a young child in September and will be refunding you your deposit. DO this after you have interviewed and replaced with an older child - at the same time unless you plan to be honest with the new ones you interview about your mat leave - and they will ask for sure at the interview what your plan is you run the risk of them leaving as soon as they find out.