Some schools don’t count 9th in their calculations. Others like to see an upward move.
If they are C or better, I would not convert to P/F. Frankly surprised the school would allow it.
There will be colleges for your child either way.
But I think you own the record vs trying to hide it. This way you eliminate any apprehension on the part of the admissions officers that something is hidden.
Just my opinion.