Played this with 2 six year olds and an adult, with another adult as GM (goblin master?). It was a lot of fun. The kids got the idea of switching and things sometimes not going as planned. They had a blast and we made it from sneaking onto the train to parachuting out in takeout boxes using a wizard's hat as a parachute.
However they were pretty cooperative, and I think for a more cooperative table you might want to adjust the result bands to make mixed success more common, since extra D10s are pretty powerful.
The end game mechanic could use a bit more clarity - does it simply become impossible to succeed? What does that mean? Should they just use the tokens to get an automatic success? I feel like they need some way to still succeed with the dice once the tokens become limited. Though maybe the ceiling is on individual dice rolls, but you can still add multiple? It just wasn't very clear in the rules.
Finally it might be helpful to have a page of inspiration for things that can happen due to failure, like a pbta GM moves list, since a low prep high improv game can have a high cognitive load.