Cooper Kids series by Frank Peretti
Recommend with some notes below.
I loved these as a kid! I don't remember how old I was when I read them, but the suggestion range on the cover is 10-14. Some of these are intense as kids books go so depending on the child some may ok earlier, but I think the stated range is probably accurate for most.
As an adult who just reread them, they aren't top tier literature. That's ok. They're still good reads & their aimed at kids. Not everything needs to be top tier.
One of my favorite things about the series is that it is solidly Christian. Not "oh we threw a prayer in over the meal so it's Christian" but scared: pray, thankful: pray, confused: pray for guidance, running for your life: pray as you run. But it's not hokey. It feels very real as a kid reading the books & is faith building!
The series is a mix of genres and this is where my age caution really comes from. If I had to categorize all of the books with one label is would be "Spiritual Thrillers (for kids)" though the adventure label definitely applies to all of them also. Breaking it out:
Book 1: Door in the Dragons Throat - spiritual thriller
Book 2: Escape from the Island of Aquarius - adventure/maybe a bit of science fiction ala Journey to the Center of the Earth
Book 3: Tombs of Anak - spiritual thriller
Book 4: Trapped at the Bottom of the Sea - adventure/"real life (unrealistic)" peril- this is the one book where you don't see the 3 Coopers getting along harmoniously & you see tensions and hurt feelings.
Book 5: Secret of the Desert Stone - adventure, definitely supernatural aspects but it's not a thriller
Book 6: Deadly Curse of Toco-Rey - thriller but not a spiritual thriller
Book 7: Legend of Annie Murphy - science fiction
Book 8: Flying Blind - adventure/realistic peril
As a kid the ones that stood out to me the most & were the scariest are books 1, 3, and 6. Book 7 was odd in a different way because of the time travel, but it wasn't scary. Book 8 was my least favorite as a kid - I almost (but not really) found it boring. That's just because they're not traveling anywhere this time. As an adult it's the only one of the books that brought tears to my eyes - it's much easier to create separation when you're looking at extremes and improbable situations like running for your life in an underground booby trapped tomb than something that feels more familiar like the possibility of an air plane crash.
If you're not familiar with Peretti please preread these books. They're pretty unique in Juevenile Fiction from my experience. If you are familiar with Peretti don't worry, he writes to kids just fine and while there are those spiritual thriller aspects theyre quite tame compared to his adult novels!
The Cooper Kids (age 13 & 14) are the only kids in the series, which I actually like & kids will too I think because they're given adult sized tasks and are responsible with them. The kids as well as thei father & various other adults pray & reference the Bible with regulararity.
The theme of the books is God is bigger, stronger, more powerful than any other religion or curse. He controls the earth. He convicts. He reveals. He works things for our good. Few kids books give that awe-some perspective in my opinion and I'm glad that these books tackle that.