I think your current children cover quite a wide range of styles so you could choose from almost anything for number 6.
My suggestions with justification of the vague vibe/theme I get from the names:
Similar to Naomi and Elias (Biblica/Christian/Jewish):
Abigail, Bethany, Delilah, Esther, Genesis, Hannah, Keziah, Lydia, Miriam, Phoebe, Rebecca, Rachel, Ruth, Sarah, Talitha, Tabitha
Andrew, Benjamin, Christian, Christopher, Daniel, David, Ezra, Felix, Gabriel, Isaac, Josiah, Joshua, James, Jonah, Jacob, Jude, John, Joel, Luke, Micah, Moses, Matthew, Mark, Michael, Noah, Nathaniel, Peter, Paul, Philip, Reuben, Solomon, Simon, Seth, Timothy, Thomas, Titus, Zachariah
Similar to Alastair (Scottish/Celtic/80s/90s):
Ailsa, Amy, Bryony, Bronwen, Bridget, Catriona, Ciara, Caitlin, Eilidh, Fiona, Hazel, Isobel, Jennifer, Kirsty, Laura, Melissa, Natasha, Robyn, Samantha, Stephanie, Shannon, Sian, Vanessa
Angus, Adam, Benedict, Bradley, Callum, Conor, Damien, Dominic, Ewan, Fergus, Fraser, Gareth, Harris, Jake, Kenneth, Kieran, Liam, Lachlan, Martin, Patrick, Robert, Ross, Richard, Rory, Stephen, Vincent
Similar to Poppy (Contemporary or Revived/Popular/Cute):
Annie, Amber, Ava, Bonnie, Bella, Chloe, Connie, Darcey, Daisy, Ellie, Ella, Evie, Eva, Emily, Esme, Elsie, Freya, Gracie, Hallie, Holly, Heidi, Ivy, India, Josie, Jasmine, Katie, Kitty, Kayla, Lily, Lucy, Lottie, Luna, Layla, Molly, Millie, Maddie, Nancy, Olivia, Ruby, Rosie, Sophia, Summer, Scarlett, Tilly, Willow
Arlo, Alfie, Archie, Barney, Bobby, Charlie, Ellis, Ethan, Harry, Harvey, Hayden, Jamie, Jonty, Jasper, Lucas, Logan, Leo, Milo, Nathan, Nico, Oliver, Ronnie, Theo
Similar to Imogen (Classic/Popular)
Anna, Annabel, Beatrice, Charlotte, Clarissa, Eleanor, Evelyn, Emma, Florence, Francesca, Felicity, Georgia, Georgina, Harriet, Isabella, Juliet, Jessica, Katherine, Louisa, Leah, Matilda, Madeleine, Natalie, Philippa, Rhiannon, Rosemary, Sophie, Susannah, Thea, Tamara, Victoria, Zara
Alexander, Edward, George, Henry, Lewis, Nicholas, Oscar, Quentin, Victor, William, Xavier, Zachary