What I would worry about more, if I still had small children is how their friend's parents keep their HG stored (or not).
But in your case, it's not like it was in the 50s, 60s and 70s when kids played with toy guns (that looked pretty real). I don't envy you having to try and train them.
Most folks here say they just took their kids to the range when they were teenagers and let them shoot .22 pistols.
You might talk to the 5 y.o. while you clean your guns and start getting them acquainted. But beware, they are now asking kids in schools if their parents own HGs and I don't think it's a good idea to be disclosing that to everyone.