The short answer is that if the chest clip happens to be a bit low should a crash happen, then it's not going to be a big deal IF the harness is otherwise adjusted correctly.
In fact, it's not even a requirement for carseats to have a chest clip in the USA. It's purpose is a pre-crash positioner, intended to help keep those shoulder straps where they should be. Obviously, we always suggest that if a chest clip is present, then it be adjusted according to manufacturer instructions.
Long term, some way of getting your kiddo to comply is likely the best answer and I hope someone else has some good tips on that for you, maybe along with the possibility of a G4 replacement Natalie mentioned! Short term for this trip, I don't think it's going to be a big deal since you said the harness is tight. If the child is rear-facing, then the chest clip is even less of a factor, due to the inherent safety of rear-facing seats that makes the harness adjustment somewhat less critical than in a forward-facing restraint.