Personally I like the name jackson. It sounds strong and commanding, and since she already has a parker, naming child #2 jackson would give them both that last-name feel.
If she wants to go with a name that has a similar sound, kyler wouldn't be bad. Parker and kyler.
And just my opinion, but the names kieffer and preston sound very ... Hmm ... "snotty frat boy". Just me, though.
Tell her congrats on her baby for me!