During my time at SMU getting my education certificate, one of the projects I did was device a superhero thematic unit. That sort of thing is totally fun, I hope you and your kids really enjoy putting it into play.
Some suggestions for writing topics: Make up and describe your own superhero; what is your origin story (ie, struck by lightning, bit by a radioactive arachnid, hit in the head by a rock)? What is your secret headquarters like? Who is your archnemesis?
Math activities: Fractions with superteams. If 4 of the X-Men get called away to fight the Brotherhood of Evil Mutants, what fraction of the team does that represent?
If Iron Man lives 1,876 miles away from the Hulk, how far away is that rounded to the nearest ten?
Good luck, and Excelsior!!