Yeah, the plot holes are huge. And as you say, the more interesting story was the humans, but they pretty much ignored that aspect. The style of the movie is that of a live-action cartoon in many ways, or so I thought.
They could do a prequel to this. Thing is, those spending money on the production might not see any value in it unless the viewership warrants it. They need to show what was going on before these events. After all, humans colonized Mars and developed an entire civilization there before returning to earth. That is an interesting story! But those plotholes are friggin YUUUUGE. Where did the children come from? How would any of these "others" (trying not to spoil here) forget what they were? Any time there was a car accident, or an accidental crushing of any of them or extreme physical damage from an accident, it would be obvious. What about history books/videos? Why wouldn't they be aware of the humans' eventual return? The dreams the main character was having would also be happening with others around the world, right? Still, the concept was interesting.