How old are your kids? It's easy to do (just bolt them - the belts, not the kids! - through the tub with a good wide steel load spreader behind), but I doubt anyone on here would advocate it.
People just aren't supposed to bend the way a lap belt on a bench seat would make you bend in the event of an accident.
Try a search on here. The alternatives and numerous seating approaches have been covered in a fair depth.........usually about once a fortnight!