6 year old cyclists are like horses. I'm not quite sure how I should pass them. (And there's at least one on most sunny days on my bike ridecommute).
If I ting the bell 6yo is likely to turn round, forget about the whole "go in a straight line" thing, but will usually stop before they cycle into the river/their brother.
If I cycle slowly to wait for the path to widen, the parent usually notices and says something like "move over Tom! A man wants to come past!". Tom will zig-zag over the path, deciding between left and right, then settle on left. I'll pass as widely as I can, and as quickly as I can, in case Tom decides he prefers the right after all.
The other option is to whizz past giving parent and child a wide berth and no time to fall off, but that scares everyone around.
So far this year I've avoided any collisions myself, but I have seen one small boy panic and crash when his mum told him to move over, one slightly older boy crash into his friend on purpose -- "Adam! there's a bike coming the other way! I'm gonna crash!" -- and a while back I crossed a hillside t-junction just when 3 boys coming down the hill on a single BMX with no brakes did too. I swerved, they swerved, I stopped, they stopped. But I did it with brakes, and they did it by crashing into a parked police van.