Not set in stone, but we are fairly sure we are going for:
Boy: Matthew James
Girl: Penelope Grace
James and Grace are family names, so we're pretty set on using them as middle names.
We know they aren't terribly unusual names (any of them). Our taste is quite classic, but we would probably like to avoid a very popular (maybe top 5) name as a first name, if possible.
Other options on our list were Philippa, Peter and Christopher.