I think you could use [name_f]Thea[/name_f] and call her [name_m]Teddy[/name_m], since both are derivatives of the same name.
Another idea would be to give her a first name that starts with T and a middle name that starts with D. That way her initials are T. D., which kind of sounds like “Teddy” which could be where you get the nickname.
With that, here are some T. D. combos for you:
[name_f]Tessa[/name_f] [name_m]Dakota[/name_m]
[name_f]Tallulah[/name_f] [name_f]Demi[/name_f]
[name_m]Tatum[/name_m] [name_f]Diana[/name_f]
[name_m]Taylor[/name_m] [name_f]Delaney[/name_f]
[name_f]Teagan[/name_f] [name_f]Dawn[/name_f]
[name_m]Tate[/name_m] [name_f]Daisy[/name_f]
[name_f]Tenley[/name_f] [name_f]Delta[/name_f]