I started doing this again this Summer when collecting the small hatlers from friends houses, cubs, etc. I keep to the real back roads though with only occasional (and ideally downhill) patches on slightly bigger roads.
The key thing I'd forgotten was the surprise factor involved when you brake. You have to have arm and shoulder muscles of steel to stop the spare bike disappearing off ahead of you.
Even with clipless this is possible, and I have stopped and set off again without my feet touching the ground by using the spare bike as support. Great fun.
The spare bikes in question are for 10 and 8 year olds, so not super small but not exactly large either.