Of course, the answer to that isn't straightforward, it depends on residence.
Presuming that child was born in the UK after 1983, they are a citizen if:
- One parent is a British citizen, or,
- One parent has become a British citizen or settled in the UK since the birth and the child is under 18 when applying, or,
- The child was born in the UK and lived in the UK until they are 10 or older
For more details and applications see Gov.uk, Register as a British citizen