Or is it ok to wait until afterwards?
Check what the arrangements are for the specific university - date by which your DC has to have applied in order to be covered by the accommodation guarantee, is the accommodation booked 'live' (you book a specific room now, not enter preferences and the university allocates at some point using an algorithm etc)? 'Live' booking inevitably means that the earlier you book, the more choice you have, other methods are more like e.g. applying for state school places, iyswim.