No, the child automatically gets British citizenship.
Because the baby boy was born while Begum was still a UK citizen, he is British, according to legal experts. The child’s citizenship is unaffected by the move to deprive her of her rights, the home secretary, Sajid Javid, indicated on Wednesday.
https://www.theguardian.com/uk-news/2019/feb/21/shamima-begums-family-hope-to-bring-her-baby-to-uk