I think Alice/Charlotte/Elinor/Emma/Isabella/Eloise are nice for a girl and George/James/Alfred/Arthur/Alexander for a boy.
Middle names could include Victoria or Elizabeth. I don't think Diana will be a middle name but if its a boy, why not Spencer as a middle name? William will also be a middle name.
Love your choice in girls's names. Can't see Emma though... not really regal... and doesn't Helen Taylor already have an Eloise... so thats taken. The others are real contenders.... Alice/Charlotte/Elinor and Isabella are gorgeous and lets not forget, its not just a princess being named... this kid will be Queen one day so it has to be a name fit for a Queen.
George topping the list for the boy's name.. love it !! Alfred and Arthur seem a bit old fashioned, can't see James being used, even though I love it... as it may be suggested that good old controversial Uncle James on the Middleton side of the family is being given a nod of approval !! Alexander ...mehhh...can't really see that. I wonder would Henry be considered as a nod to Harry and Charles could be up there as well as a favourite with them.