I'd say the overall feeling of the examples you've given (and the feeling I get from most first books in MG series) is 'this adventure is over, but there are other adventures out there'. So there's a feeling of closure, of satisfaction that the problem you set up in the book is resolved within the book, but also the expectation that there are possible further stories that might come out of it. If your book does that, if the whole book the characters are trying to solve problem X and by the end they solve problem X but oh no there's the hint of problem Y looming on the horizon, then that's okay.

Ask yourself, if (heaven forbid) this was the only book that ever got published in the series, would it be satisfying? Would a reader finish it and say 'Well it's too bad they didn't continue the series but that was a pretty good story' or would they say 'What? That's how it ends? How come they didn't finish the story?'

If you're setting up the Big Bad the whole time and by the end the heroes haven't actually stopped the Big Bad at all, then I would feel unsatisfied. If, however, the goal is something like 'Let's free our kingdom from Minion Dude!' and they free the kingdom from Minion Dude and have a big party, then I'd feel okay with it even if I knew there was a bigger bad behind Minion Dude looming off in the distance. It wasn't ever their goal to defeat Big Bad (at least not in book 1), so they accomplished their immediate goal and see the immediate rewards. It's all about how you set up their adventure and talk about their goals. Don't make Big Bad the focus. Keep things close and immediate so you can manage reader expectations.