Okaaaaaaaaaaaaaaaaay. I'll comment on your names and then try and add suggestions.
Corin - I like this, but it does sound quite like a sur name to me.
Caleb - Love this one.
Roan - Iike this one. DOesn't immediately strike me as being a good match for your girls' names though.
Cassius or Cassian - I Like Cassius, and I like Caspian as discussed on this board a while ago. But not Cassian. It sounds feminine. Like Cassie-Anne. Julius is a brilliant Roman classic that matches your girls' names very well.
Jude - Quite like this. It is slightly poncey! but it's still a good name.
Reuben - Really love this one.
BoYS' names I can see with your girls. Dominic/Dom, Jeremy/Jem, Benjamin/Benjy, Jonathon/Jock or Jonty, Robert/Robbie, Richard/Richie, Gregory/Rory, Thaddeus/Teddy,Thaddy or Theo
Charlotte - nn Coco - I really completely totally love the name Coco (as Lola would say) and I love Lottie too. Not tremendously keen on Charlotte tbh, but I think the long name/short name does blend in very well.
Cordelia is a very versatile name with so many nns you mgiht like. Coco obviously, and Didi and Delia and even Celia !
Beatrice - nn Bea - I like this one. Also Bibi is lovely. Beata (pronounced BeyATa is a germanic nn option, nms tho!)
Cecelia - nn Cece - quite like this but too many Ss. Perhaps? I like Cecily though.
Olivia - nn Liv - Like it. Liv is very pretty, but it sounds a bit weak next to Lexie. Lexie and Liv. Lexie just sounds so much stronger.
Here are some more classic names with very pretty nns.
Margaret - Maggie or Greta
Penelope Poppy or Polly
Susannah Susie
Jemima Jemma
I might have more later! Are any of these suggestions on the path ?