You do not include the child in IMM0008 in the relevant section because the child is already a citizen. (If you read the instructions carefully it says this - 'you must include ... all your dependent children ... who are not already permanent residents or citizens of Canada'). This particular form should include only those who are applying to become PRs (or could i.e. not accompanying on this app).
Note, there are other forms eg additional family information where you would include.