This is from the instructions for the I-485, available on the USCIS website:
Filing Fee :
$325.00
Special Instructions :
Note on Filing Fee:
Original fee plus $70.00 biometrics services fee if you are 14 years of age or older. If you are under 14 years of age, the fee is $225.00 with no biometric services fee. If you are 79 years of age or older, the fee is $325.00 with no biometrics services fee.
If you are applying for permanent residence based on having been admitted to the U.S. as a refugee, there is no application fee; however, applicants age 14 - 79 will still need to pay a $70.00 biometrics fee.So the answer depends on the age of the child or children. If the child or children is/are under 14, you pay $225 AOS fee only (which is what we did with my stepson and had no problems). If the child or children is/are 14 or above, you pay $325 AOS fee, plus $70 biometrics fee PER CHILD.
Best wishes,
Maya
PS It looks like you first asked a question in the original post, and then answered your own question incorrectly. Or am I missing something? Maybe I should go have a cup of coffee...

PPS Now I see you are possibly in the wrong forum... yikes... <looking for coffee cup>