Mary Grace and William sometimes play really well together. Sometimes, however, it doesn't go so well. Imagine observing this:
Mary Grace is playing with her Ponyville. She has Pinky Pie and Minty and Star Song and the others. They're having a birthday party for one of the ponies. It's a great party until William joins in. William brings his dinosaur to join in the fun. Of course, being a dinosaur, the first thing it wants to do is jump around and knock all the ponies over, disrupting the party. The dinosaur proceeds to get into an epic battle with one of the ponies, at which point Mary Grace is running crying for a parent to intervene.
She calls this "playing naughty", as in, "No William, I don't want you to play naughty!"
Now imagine the other side. William has a franken-toy setup involving trains running over dinosaurs, blocks acting as the bowser trying to beat everyone else, and even pull-back cars running through the whole disaster. Mary Grace brings her Barbie doll into this, suggesting they could all work together to throw a surprise party for another Barbie.
There's no term for this intervention, but again, it ends up in a brawl of some sort requiring parental intervention.
Sometimes they play well, other times not so much!