While the noise level is a definite improvement over the venerable CC130-E/H, I would highly recommend hearing protection for a child ..... furthermore, I would recommend it for adults as well.
The noise in the cargo compartment is a function of the recirculation fans and also the environmental systems. It is a high pitched hiss. There's plenty of noise from other aircraft related systems, but the ECS and fans are the primary noise maker in the back.
Noise studies have revealed that the loudest part of the cargo compartment is actually close to where the loadmaster's staion is, towards the front of the cargo compartment, and the quietest is towards the rear of the cargo compartment near the ramp hinge (between the para doors).
Either way, EVERYBODY should wear hearing protection when occupying the cargo compartment for protracted periods of time.