So when I started the game with my kids, each played one character. They chose a wizard, cleric, and rogue. Realizing they needed some muscle I played the dreaded DM PC. A dwarf barbarian. To keep things on the up and up, and not to take center stage, I used Charisma and Intelligence as dump stats. He was basically a lump of dumb muscle. He made no decisions other than what to hit with his axe (hint: whatever was in front of him). He talked to no one. He's become a loved and valued member of the party.
As they became better players, I introduced some henchmen, which they each took over, a druid, ranger and a fighter. Last session, another NPC accompanied the group, giving me two characters to run, and swelling our party ranks to 8. Too much for me to do, and hard to challenge a group that size.
So I told the kids that I wasn't running a character any more. He would be available for one of them to run, and each could continue to run two characters at a time. Both Rurik and Filch are available for them to play, and any character they place in the pool for someone else to use. As time goes on, other characters can be introduced to the pool, and used as needed. Kind of like the Avengers.