Complex sentence: But over time both notice that the longer the star stays on earth, the weaker it becomes (with its colour fading while its powers fail), with their next door neighbor Max they find a way to send the little star back into outer space.
Split into simpler sentences: But over time both notice that the longer the star stays on Earth, the weaker it becomes. The siblings and their next-door neighbor Max eventually find a way to send the little star back into outer space.