Coming up with a name for W&C's babies will be difficult! Philippa/Philip and James have already been used for William's cousin (James Alexander Philip Theo), but maybe they'll go with something different.
boy:
Edward Francis Arthur William
Edward - family name of the Windsors; honors William's uncle, the Earl of Wessex as well as the eight kings named Edward that came before his grandmother, the Queen.
Francis - Michael Middleton's second name, and is the male equivalent to Diana's second name of Frances. It's also the first name of Diana's mother.
Arthur - one of William's given names (also one of Charles'); it also honor's the prince's great-great-great-grand uncle, Prince Arthur, Duke of Connaught.
William - after Prince William and it's also James Middleton's second name.
girl:
Cecilia Frances Elizabeth Mary
Cecilia - the given name of William's maternal great-great-great grandmother, Cecilia Gordon-Lennox
Frances - Diana's second name (also the female equivalent to Catherine's father's second name)
Elizabeth - for the Queen, the late Queen Mum, Duchess Catherine's second name as well as her mother's.
Mary - for the late Queen Mary; it is also the given name of several of Catherine's ancestors.