I watched this film a few times in the early 90s, was talking about it with my brother today and how much we enjoyed it at the time but neither of us can remember what it was called!
It was about s group of teens in the future where water is a valuable commodity and everyone is kind of poor and grubby. The main characters were quite stereotyped. They lived in a children's home/orphanage in the desert. One of the boys is a very young teen so gets sidelined and patronised a lot by the older ones. One of them is a pretty girl who claims to be a princess but none of them believe her. The people running the children's home are really unkind, doing things like making the kids dig in the desert, shouting at them, that sort of thing.
The kids find a glowing ball/orb, it's special and is going to help them fix everything, bad guys are looking for it so they have to band together to protect it. Youngest teen has the most attachment to it. They go on a journey with the orb into the desert but we can't remember why or where.
The pretty girl who claimed to be a princess disappeared part way through the quest and shows up again a little while later as a masked/robed person in the desert, she has a hosepipe and sprays water at the main teen boy/love interest. It turns out she actually is a princess and her tribe found her while they were out in the desert, she says she'll use her new position to help them all.