I would definatley take a seat for a young child. A child under 5 is not old enough, large enough, or mature enough for a booster and they are not allowed as a seat on the airplane. The child will be more comnforatble and far more safe in a car seat.
When I went this past May, I left ds's seat behind (he was 4.5) since we would only be using disney transportation, and you cannot use car seats or boosters on the bus. Ds's legs hit the seat weird and he ended up kicking the poor man in front of us alot. The seatbelt also did not tighten well for his small size. This trip he will have his airline seat, the radian, which also folds for travel.
Many folks also strap the carseat to a luggage cart like the kind you can get at
walmart.
If you purchased a seat for said child, then the airline has to allow the use of an approved car seat. These seats can even be used rearfacing (which is safest for young children). The seat will need to placed next to the window, an dthere are a few rows that you cannot sit in, but they should relocate you. A carseat is safest , especially for such a young child.
If you do not purchase a seat for this child, then the airline may allow you to bring a seat if there is room. Just know that lapbabies are placed under the seat like luggage in the case of emergencies, ugh. There has been a move to require seats for all children, especially after several preventable deaths and injuries for children that were unrestrained during turbulence or emergency landings.
Hope you have a safe and fun trip! Time to pack for me!!!