Here's another great offering from the BBC. At Digger and The Gang, there are three comic templates, and kids can simply click on each speech bubble to write their own words. Text needs to be short - about 20/30 letters and it runs out. Some frames are clickable, meaning the picture can be changed. Stories are about an underwater, jungle or city adventure. You don't need to register/log in, and you can print out your creation.
At the main website, kids can also read about the characters the templates were designed around, and enjoy other activities like mixing music, creating secret messages or reading comic style animations about the gang's adventures.
Some other comic creators I've recommended are Comic Master, Stripcreator, Beanotown, Creaza's Cartoonist, Story Maker, StoryJumper, Make Beliefs Comix, Toon Doo, and Myths and Legends.