My 6 year old has an abscessed baby molar. Is it best to extract?

INFECTED MOLAR. The dentist will first give prophylactic antibiotic before doing extraction.
Yes. If a primary or baby tooth is abscessed the best tx is always extract. Then because of your child's age you will need to do a space maintainer to reduce space loss after. Most important is how did it get that advanced with out detection. Please see your dentist regularly to avoid these complications in the future. In almost every case they are preventable with good hygiene and routine visits.