Yes you need to inform them, it will not affect in any ways, provide all your details in reply to p1 and mention that you have change in situation and details of your child and attach his or her birth certificate, i have similar situation and i got p2 on 0ct 8, waiting for ecoprHi
I have received p1 today and recently we gave a birth to baby so should I mentioned in reply for p1 is that okay?