Years ago I read a book called Peace Like a River by Leif Enger. It's been a while, so I don't remember everything, but there are two siblings in the book, a brother and a younger sister. The sister writes an ongoing poem throughout the book which reflects the family's problems in a metaphorical sort of way. At a critical point in the book when the family is in dire need of help her brother begs her to kill off or otherwise get rid of the villain in her poem. His sister bursts into tears. She says she's tried and tried to get rid of him, but he keeps coming back. She couldn't make evil disappear.
I think this is at the center of your question. The characters in your book have solved the immediate evil in their story, but a greater evil exists - one they can't get rid of. Later books may reveal how they build a life where they are, or manage to escape the continent, but the greater evil will continue to exist. It's something we all have to deal with.
I was completely satisfied with HP1, because the immediate resolution was so strong. I also didn't know there were sequels planned. It wasn't until the end of HP2 that I realized there was an ongoing story arc that covered all the books. For the first two books I thought the series was going to be episodic, but I was satisfied with each book as I read them. Even after Harry vanquishes Voldemort, you know another foe will rise, because they always do. Turns out Big V was just part of a grander scheme.
This is a long winded way of saying I think your story will fly as a stand alone as long as that immediate resolution is strong enough. Problems will always exist in our lives. You do leave the kids in a bad place, though. The pull to go home is ancient and incredibly strong. To cut them off completely is going to be a bit hard to swallow. This first book sounds like it is absolutely crying out for a sequel.
Stars Wars Episode IV is another good example of a story that could have stood alone because the vanquishing of the immediate evil was so completely satisfying, but the Empire, or Empire stand-in, always finds a way back.