This was a book where several kids who died in several historical disasters were given a choice by an old man about if they wanted to die in that disaster or come with him and be sort-of "time police", the only catch is that they would be locked into that role. There was a boy who was on the Titanic, a girl who was on one of the 9/11 planes, and I think one or two more.
They are taken to this secret hideout which is in New York during a time loop of 9/11. When they are there, every day is the same day, September 11th 2001, and the reason for this is because it keeps them away from prying eyes. The current timeline, not the time loop, is in the future, where time travel technology is invented, and the old man and the kids serve to stop anyone from misusing time travel.
In the current time, the future, a group of heavily armed Neo-Nazis raid a museum, maybe the natural history museum?, to abscond with the first time machine, which is on display, so they can go back and have the Nazis win World War 2. The Neo-Nazis succeed in their goal and go to Hitler's bunker in the Winter and show him all of their future technology.
This book was part of the series, and the second book concerned dinosaurs.