I second, "Fantastic Planet". I'd also recommend a Jules Verne-based piece from the '60s. Not conventional animation, also sort of a collage from various Jules Verne stories/novesl. Sorry I can't remember the name ("The Amazing World of Jules Verne"???).
More for kids, consider, "Astro Boy" and "Beanie & Cecil" if these are now available.
More for kids, consider, "Astro Boy" and "Beanie & Cecil" if these are now available.