A booster seat is not safe for a two year old child. Really, 4 years old AND 40 lbs is the minimum for a child to be safe in a booster.
A booster seat, to be safe, requires a child to sit up straight without slouching, not mess with the seatbelt, lean forward, and stay in the proper seating position for the entire ride.
Can you bring your own harness carseat from home?