There has to be a seat with a seatbelt for every passenger. A passenger in this context is a person age 2 years and above, hence a person under 2 years old (infant) does not require a seat. These are the rules, and there are no way around them.
If you have a child 2 years 6 mds, and you buy an infant ticket (below 2 years old), you risk being offloaded since it isn't a valid ticket. On the other hand if you buy a ticket for a child, the airline has to provide said child with a seat.