My child can't get their arms out of a harness
There comes a time when a child can get their arms out of a harness, they become strong enough and clever enough to do so. There are a couple of devices on the market specifically designed to stop them doing this, it is far better to get one of the devices than improvise yourself. The devices are easy enough that anyone can undo them should you have the need in a collision. If you improvise yourself by tying them together, someone else may not be able to undo them when time is critical so try not to improvise.