To start creating from scratch, this guide may be useful: How to Create a Comic Strip With Your Kids in 7 Easy Steps. It works through from getting inspiration, creating characters, writing the story, creating the borders, doing the lettering, penciling in content, making it permanent to the final completion.
Education .com offers useful comic strip writing templates.
Scholastic offers a template for a comic strip based on the Wimpy Kid.
Imagine Forest offers ‘How to Create a Comic Strip in 6 steps’. This is the basis of a three frame story which may be great for the very young, or to introduce comic writing to older children. This site also encourages users to sign up for Marty the Wizard : “Marty the wizard is the master of Imagine Forest. When he’s not reading a ton of books or writing some of his own tales, he loves to be surrounded by the magical creatures that live in Imagine Forest. While living in his tree house he has devoted his time to helping children around the world with their writing skills and creativity.”(Imagine Forest)
Choralsongster shares, through the TES, a short PowerPoint presentation to introduce comic strips to children. This Looks at the different features of a comic strip, such as: caption boxes, speech bubbles, thought bubbles and action words. It also links well with the topic of Superheroes. An ideal topic for comic writing!
Frugalfun4boys is offering a Printable Comic Strip Template with story starters leaflet for the sake of signing up.