Parodying posters in general are not the problem, its the work they chose to parody here. The Handmaid’s Tale is a story about a repressive totalitarian state that forces women to become sexual slaves in order to repopulate their dwindling population. The main protagonist is one such of these women and she is subjected to rape to bear healthy children.
If they chose to parody something else, it would have been okay. But the Handmaid’s Tale is just too dark and depressing to be parodied or made funny like Warhammer 40K, let alone in a work directed for kids.

And it's getting backlash for that. Good