Reddit mentions of Chasing Vermeer (Scholastic Gold)

For me, the dividing line almost always lies in the main conflict.

For MG, the main conflict is home-based. By this I mean, whatever the goal of the main character is, it is directly linked to his/her home life. It could have larger repercussions, but the hero is in it for his home. Chasing Vermeer is a good example of this: the kids are solving an art mystery that the entire world is focused on, but they're solving it to save their own community.

For YA, the main conflict is world-based. The main character is doing something broader than him or herself or things linked directly to his/her life.

Another way to put it: MG is about the main character finding a place within his home. YA is about the main character finding a place within his world.

Really, this is the biggest dividing line to me--everything else is flexible.

Blue Balliett's series:

1. Chasing Vermeer
   
   
2. The Calder Game
   
   
3. The Wright 3
   
   Stephen Hawking's series:
   
   
4. George's Secret Key to the Universe
   
   
5. George's Cosmic Treasure Hunt
   
   
6. George and the Big Bang
   
   
7. George and the Unbreakable Code
   
   Others...
   
   The School Story (It has two female leads, not sure if that will be a deterrent for your nephews)
   
   Einstein Anderson series
   
   Encyclopedia Brown series
   
   Hardy Boys series
   
   Sammy Keyes series
   
   Check the ages. All of these are above a 5-year-old, unless you're reading to him. Most of what you're asking for will end up being detective mysteries.
   
   I wouldn't recommend nonfiction unless the subject matter matches their real-world interests... or you happen to know they prefer non-fiction.