I think Poppy's adorable!
Cute surnames that work on girls:
Leigh or Lee
Gene or Jean
Kel by or Shelby
Kinsey or Kenzie
Kit (as a nn for Katherine, Katrina, or on its own)
Sandie (nn for Alexandra, or on its own)
Whitney, Whitley, Whitby
Some of these names, like Devon or Wylie, might sound a bit more 'out there' than others, but I've come across them as girls' names.
How about Marlow Beatrix? nn could be Mae or Maeby or Marly Bee or Arly Bee or Arlo....
Harrison and Shepherd both have an S and an H - you could do a girls' name that follows that pattern as well - Ailish, Thistle, Persephone (nn Poppy!)....
I also like Carys or Willaby or Phoebe (nn Bea). Good luck!
I immediately thought of Waverley for you, which I believe someone else has already suggested. Different, but not weird, sort of surnamey feel like your sons...I think it's perfect!
Other ideas for you: (feminine, different, not too out there...)
Meredith (has that surname feel)
Cassia or Cassidy
Good luck finding something perfect for your little one! Congratulations.
I immediately felt names that were naturey and girly were the way to go...Some of my favourite names seem to fit well...
Marigold (Goldie... love Goldie!)
Magnolia (Maggie, Nola, Mia, Gia, even Goldie would work again!)
Also 'red' names come to mind for some reason :)
Scarlett (Lettie, Etta)
Valentine (Tiny, Leni... It's fits the surnamey theme)
Ruby (probably too popular but I like it with your sons names!)
Poppy is a great fit with your sons in my eyes. I do like Isla but it doesn't feel so unusual)
Garland or Astrid were the first names that popped into my head. I love Garland. It brings to mind images of Judy Garland. And I just think Astrid would be obviously girl, but it stands up nicely next to Harrison and Shepard.