For a boy, either
Matthew Preston or
Alexander Preston. If you use
Alex, he'll spend his life explaining that he isn't 'really'
Alexander, so he might as well be
Alexander and have a choice of nns if he ever wants or needs a different one.
Kirk is too out-there, considering your existing children.
For a girl, definitely
Andrea Claire. No contest; the others just don't fit with the names you have used.
Andrea Melanie could also work.