I would think the child would need to be able to sit unassisted. 6 months might be a little young though. We took DD biking around 12 months and rented a bike trailer. She wasn't comfy in it because she kept slouching.
I did see another family using a bike seat that sat between the bike handles and the rider. The child was around 15months and seemed happy.
Make sure you get an infant helmet and get the child used to wearing it before you attempt to go biking.