was going to suggest the same thing. i have the aces races daytona seats in my rzr, i like them. i also use their booster cushion for my 10yr old daughter. she likes the booster too because when we are at the hill or somewhere just taking a break she takes it out and uses it like a chair in the sand.
(1) Booster Cushion 3x2 – Aces Racing (aces-racing.com)
depending on size maybe move your older boy to the booster cushion and then give the younger one the other seat.
ill add that aces racing has been a great company to deal with. good products, cant beat the prices and great customer service. at least in my experience