Congratulations!
Charlotte and Florence Jack
Noah and Theo Jack
Florence and Theo Jack
Are the best combos I think - all nice
Daisy, Betsy and Charlie Jack sound like appliances and Anna Jack is too sharp. Jude, Joe or Tom Jack just sound daft - like double first names (Thomas / Tommy and Joseph will always be shortened).
Jack is a nice but quite challenging surname to match, I think you need softer sounding names of 2 or 3 syllables, avoiding a y end (unless the name is very soft), and nothing that would easily get shortened to a single syllable.
Maybe -
Miles, Nathan, Reuben, Laurence, Robert, Elliot, Elijah, Edward, Henry, Silas, Alistair, Alexander, Rufus, Barnaby/Barnabas (Barney), oliver, Lucas, Marcus, Owen, Hugo
Alice, Edith, Sophie, Louise, Chloe, Phoebe, Frances, Flora, Sarah, Constance, Clara, Emmeline, Caroline, Marianne, Meredith, Evelyn, Rachel, Sophia, Claudia, Eleanor