She might want to make sure, if she possibly can, that she moves end of March/beginning of April.
As others have said, she will miss the initial round of applications and be at the back of the queue in the initial allocation.
However, if she can be in situ and with her application in by allocation day (16 April I think, but check your area) she will be able to be considered in the first round of 'late' applicants. What happens after allocation day is a massive shuffle about. Some people have gone private and turn down places. Some people have already moved. This means that various schools have places open up and take people from their waiting lists, which in turn frees up places at other schools. This is particularly true in fairly urban areas. She wants to be in at the start of that process if at all possible, so she benefits from as much of the post-allocation movement as possible.
Then, if she can, her best bet is to move close to a school with plenty of movement (you can usually find this out from the school and onto the doorstep).
The other option, which mostly applies to rural locations given the spike in birth rates, is to move close to a village school which is generally undersubscribed (again, she can find out from the school). If the school isn't full she isn't penalised in any way for applying late - they have a place, she applies (and no one else who qualifies more highly has applied for it) she gets it!
Also bear in mind that her daughter doesn't have to start school (or be home schooled) until the term after she turns five. If she's young for her year, that might give her some time to wait for a place to open up at a school she likes.