Welcome to FT sarafina01.
Your ticket does not to be canceled. As jimbo99 states, it possibly can be changed and day changes might be cheap (unlike routing changes).
If the ticket is not flexible, then you either have to buy up to a higher fare or simply forfeit the return portion.
Your 'new' daughter's ticket however will have to be re-issued and that will attract cancellation fees. If she's still an infant, these will be just a fraction of your expenses.
I had to change my returns from TPE numerous times and I always found the staff very supportive and understanding with such demands.