Thanks Eponasrider! Yeah, the forums here are a great place. Christopho has a youtube tutorial series I highly recommend, and definitely join the Solarus discord. It's a fairly active community there, much more than the forums here. I recommend messing around with Solarus for a little bit to get comfortable in the editor. Download the free resource pack, which has tilesets, enemies, and I think some items. Make a room to get used to the map editor, then make a town to get used to adding NPCs, teleports, maybe a little bit of map scripts. Then maybe make a small dungeon, but don't try and create and new mechanics, just use what's easily available to you.
After that, I'd recommend planning out a small "test" game. Maybe the hero has to go from her town to a dungeon and back, there's a couple items to find an equip. When you realize you'll need to create menus, enemies, design dungeon puzzles, figure out items, all of that, a small-seeming game like that is actually quite a lot for an amateur developer like us!
I imagine if you're a professional developer you might be familiar with agile methods and scrum boards and other tech kinda workflow strategies. I actually use quite a bit of that just myself to keep track of things and keep myself on track.
Also, I'll send you a link to join the beta! I'd love to hear what you and your son think! : D
(I'm headed out of the house right now so i'll send it later tonight)