Your children have beautiful names.
GIRL
Marlowe - I quite like it. I don't think it matches your other names (unless your Elliott is a girl)
Elouise - like it, prefer the spelling Eloise
Madigan - never heard this. Sounds like a cross between Madeleine and Madison. I prefer both of those to Madigan but it's ok. As it's a family name, I'd maybe stick it in the middle.
Florence - quite like it.
Violet - not keen. It looks and sounds too much like violent to me.
Rosie - like it but maybe not fancy/'cool' enough to go with your others.
BOY
Harvey - like it. Not sure it goes with the other names.
Frankie - not keen. It's too nicknamey for me but I really don't like Frank or Franklin. I quite like it for a girl, short for Francesca.
Sebastian - quite like it.
Atticus - really like it in a trendy, 'different' kind of way. And love the literary nod.
Darcy - I like Darcey for a girl. I've never met a boy Darcy. It's quite nice though. I just think people would assume he is a girl.
Jude - quite like it.
Other ideas - Adelie, Clementine, Elliot and ...
Bronte, Bronwen, Daphne, Danica, Francesca, Fenella, Giselle, Georgina, Hermione, Hallie, Imogen, Iris, Jessamy, Josie, Katrina, Karensa, Lorelai, Larissa, Miranda, Melinda, Natasha, Naomi, Octavia, Ottilie, Phoebe, Philippa, Ramona, Romilly, Saskia, Sylvia, Tamara, Taryn, Verity, Vanessa, Xanthe, Zeryn
Benedict, Bruno, Dominic, Dylan, Flynn, Fraser, Harrison, Hugo, Isaac, Inigo, Jesse, Jasper, Kristian, Lewis, Lorcan, Micah, Milo, Nathaniel, Noah, Owen, Oscar, Patrick, Parker, Quinn, Robin, Rowan, Solomon, Silas, Tian, Theo, Vincent, Wilfred, Xavier, Zachary