This summer my boy was big enough to sit on a bike... 1 year 3 months.
So I decided to upgrade my bike with an extra seat for him...
Pros: Extra weight in front helps to climg steeper places.
The boy sees where we go and sort of participates :)
Position between my knees and hands feels safe
Cons: No sleeping for both of you!
Warning: Kid has to be capable of holding his weight!
Costs - 2$ for metal loops which hold the chair and pedals.