Theodore is lovely, and Teddy a sweet nn.
I really like Seth.
But if I was Jackson (or worse Jaxon - don’t do that to him) or Reggie and my brother had an amazing name like Theodore I’d be seriously pissed off.
Nathaniel nn Nate, Tobias nn Toby, Gabriel nn Gabe, Christopher nn Kit, Nicholas nn Nico, Zachery/Zachariah nn Zach, Alexander nn Xander, Sacha or Sandy, Laurence nn Laurie, Robert nn Robbie or Robin, Francis nn Frank, Alistair, Silas, Rufus, Jude, Jerome, Patrick, Guy.
Your girls choices are lovely, but popular to the point of being overused - Dorothea nn Dora or Dorey, Alexandra nn Sasha, Christabel, Adela, Serena, Saskia, Cassia, Belinda, Francesca, Fernanda, Estella/Stella, Louisa, Frances, Constance, Esther are less popular alternatives