If you have someone strong enough to lift the seat with the child in it, have the child "tighten" the straps to where they seem to prefer it, then have strong adult pick them up and turn the whole seat/child combination upside down over something soft like a couch or mattress. The child will likely hit the harness hard enough to jolt them, but not injure the child.
Then adjust the harness correctly and flip them over again. The child shouldn't move at all. Point out that this is what will happen in an collision, only the collision will be much worse. Explain that the harness is what holds their body in place and protects them and that to do its job properly, it needs to be snug.
This demo was very effective with my almost 4 yo. He doesn't complain about the harness being snug anymore.