According to Article 5 of the Law of the People's Republic of China on Resident Identity Cards, citizens under the age of 16 who voluntarily apply for a resident identity card shall be issued a resident identity card valid for five years. Article 7 stipulates that a citizen shall, within three months from the date of reaching the age of 16, apply to the public security organ where his permanent residence is located for a resident identity card.
Citizens under the age of 16 shall apply for resident identity cards by guardians. Article 10 stipulates that to apply for a resident identity card, a registration form for applying for a resident identity card shall be filled in and a residence booklet shall be submitted for examination.
According to the above regulations, you should register your child first, and then apply to the local police station with your hukou.