I watched this movie as a kid. It is about two kids (brother and sister) who are from outer space but somehow stranded on earth and have lost contact to their parents.
I remember that they wanted to get to some kind of alien spaceship launch in time but barely missed it which was very sad. I think this was how the story ended so maybe there also was a part 2? Or was it a TV show?
I would guess it was from the 90's or 80's.