I think Rosa and Robin or Jonty would go well with your other children's names. You could also call a girl Robin if you spell it Robyn.
Other ideas. Nina, Greta and:
Anya, Alice, Anna, Astrid, April, Bella, Bonnie, Bethan, Bronte, Cara, Clara, Cerys, Callie, Dana, Dora, Ella, Eva, Emma, Elsa, Edith, Esme, Erin, Ffion, Flora, Hannah, Hazel, Helen, Isla, Iris, Ivy, Isis, Josie, Janet, Katie, Laura, Lara, Lana, Lila, Lily, Layla, Lilith, Lucy, Lois, Leah, Lacey, Louise, Mary, Marta, Molly, Millie, Megan, Orla, Pippa, Polly, Ruby, Rita, Rosie, Sara, Sarah, Sophie, Seren, Sasha, Sylvie, Stella, Sadie, Thea, Tara, Tessa, Una, Vera, Willa, Willow, Xanthe, Yasmin, Yvette, Zara, Zia.
Adam, Aaron, Aiden, Christian, Charlie, Casper, Cosmo, Dylan, Daniel, Ethan, Ewan, Fergus, Felix, Fraser, Harvey, Isaac, Jamie, Jacob, Lucas, Luca, Liam, Marcus, Oscar, Owen, Patrick, Peter, Quentin, Reuben, Stanley, Thomas, Titus, Victor, Xaver