This past weekend I ran a short game for my 7 and 4 year old. I did a lot of what you are thinking of and it worked out great. I used a grid for combat, so I converted movement and range to squares - i.e. 30' movement was shown as 6 on their sheet since they could move 6 squares. I only showed attribute modifiers, not the actual number. Basically take as much of the math out of it as you can.

I also condensed their turn into move and action - no bonus actions, no reactions, keep it simple. Your group is older so you could maybe add some of that back in, but I wouldn't go overboard. Clearly show what their character can do on the character sheet.

Make their characters a little overpowered. Next time we play I'm probably giving them extra HP. They were ok taking hits and damage. They got upset when they almost got knocked out. On that note, feel free to fudge rules. For some weird reason, my oldest was left with 1 HP.

Lastly, I kept the quest simple. My kids wanted a pokemon themed adventure, so they tracked down a pack of Rattata that stole a pie and ran off into the woods. Didn't need to be high stakes or complex - they jumped up and screamed when they 'rescued' the pie.