Green to mature and 'bomb proof'

Hi everyone,

Looking for advice on how to bring a green broke horse along to eventually be a horse that is essentially bomb proof and safe for children.

I have a 4 yr old gelding that was just trained over the summer. He's doing great and is making progress on each ride. However, I obviously would not put my children on him for more than a lead line ride. Down the road I would like him to be safe for my kids to ride. They are currently 1 and 3 years old so in my opinion I have plenty of time to bring this horse along before they will be ready for him.

Over the next several years what are some things I can be sure to expose him to or teach him? I'm looking for him to simply do trail riding.