If it were me, I'd try moving a trumpet. Or make an open invitation to the entire band. Moved a sax player once. Obviously, depends on kid.
I've marched bass clarinet, but that won't be enough sound alone. We were successful at concert contest once with 2 bass clarinet, a contra-clarinet and no tuba but I doubt you could march a contra. And again, probably not enough sound.
Bass guitar or synth is an option I've seen.