If the child born before you acquired PR was born in the UK, then s/he can be registered using
Form MN1. Make sure to read the guide, as that will likely answer most of your questions about filling in the form (or at least help you ask good questions when you have them). The payment slip for the application is the last page of
this document. The preceding pages tell you how much each application costs (registration through MN1 is currently £973).
You might also consider using a
Nationality Checking Service (NCS) to submit your child's registration application, as it will allow you to keep the original documents while the application is under consideration. If you use this service, you can apply for your child's UK passport at the same time (which will add the cost of the passport application to the fee). Otherwise, you will have to wait until after your child's registration certificate arrives to apply for his or her passport.
If the child born after you acquired PR was born in the UK, the s/he is already a British citizen, and you can
apply for his/her passport directly using the online form or by getting an application from the Post Office. You will need to send the Passport Office your PR card as well as the child's birth certificate showing that you are his/her parent, plus any other documents requested in
passport application guide. If your child was born after you received your PR, but before the date on your PR card, you will probably also need to send the letter that confirms the date that you received your PR.
