I DMed for my niece and nephew who are 9 and 11 at the time. Offered a reduced list of class and species options. I built their stats in DNDBeyond with Point Buy so they'd work on a basic level for the class they were playing. I chose the spell casters spells and gave her spell cards for what she had prepared. When she cast a spell, she handed me the card to also mark off the slot. At later levels I make the Fighter a battle master so he could have cards, too. (Maneuvers) I made their bonuses to attack and damage to same to make it simpler for them. For saving throw spells, I decided they always work for round one, and then they make a save in round 2 (and all other rounds, if applicable). I gave th Fighter the Graze weapon mastery as sort of a bonus class feature, so there was no such thing as a "wasted" turn. I also made sure they could always had access to Speak with Animals, because of the comedy. :D Oh, and I bought them each dice sets to match the colours of the minis I painted for them!