You can bring them on as carry on's and stow them but the kids can't sit in them. Booster seats (or any CRS without an integrated harness) are not approved for airline travel. If you want to use a CRS on the aircraft check for a sticker on the back indicating it can be used onboard, the FA's will!
IMHO if a child meets the height/weight requirement for a backless booster then they meet the height/weight requirement not to need a CRS on the airplane.