Sounds like you are doing a good job.
Yea, He's pretty large for a 3.5 year old. not fat just big. and very active. I joke that his mommas family comes from the land of giants. I don't let him have toy guns ( guns are not toys in my home). the times he has seen and we have interacted with firearms together have been very controlled. He's just not there yet mentally to really understand what its all about. That said. I don't worry because I stay on top of it.
The Fas1Safe is neat because of the way it presents the firearm and that it is quite a bit more secure then the other similar products on the market. Bad side is that they want twice as much $. The one I'm building has the same lock and is made from 1/4 instead of 3/16
Post up progress or finished pics when you get a chance.
The size of kids surprises me sometimes but my son is in the bottom 5th percentile in size, daughter is down there too. He is probably in the 95th percentile when it comes to toughness though lol. 3 is young for them to get all of it, but I did start teaching them not to touch, etc, and adding to it as they got older. Shooting skills need a lot of work, but the safety aspect they have down pat at this point. just start with the basics, dont touch, etc, then start adding to it that they can with permission, safe directions, etc, and they'll know it before their alphabet.