Kids used for time heist

Question

Book I read a few years ago. Main plot points:

• Time travel had been attempted by a research group, but was unsuccessful. They threw out the device and the janitor, who "just happened to be a quantum physicist" (I remember this line clearly from the book).

• The janitor started a group to go back in time and steal items for him. I only remember one specific item: a flag from 2076. I think that the group was composed of children and they may have eventually rebelled against him.

• People who stayed in the past for too long suffer from "time sickness"

Answer 1

This hasn't collected a good answer yet, so I'm going to propose a partial match. The Missing series by Margaret Peterson Haddix features a group of young people (in the first book most of them are thirteen) who are recruited by a time-travelling organization to carry out missions in various times.

The person who tries to recruit them (Jonah, the main viewpoint character, his friend Chip and sister Katherine) is posing as a janitor when they first encounter him, and thereafter (even through succeeding books) they refer him as "JB" (short for "janitor boy") since he won't give them a name.

The first time they are sent through time we are introduced to "timesickness" as both Jonah and Katherine are hit by it:

"Katherine?" he asked. "Are you all right?"

"No," Katherine groaned. "I think I'm going to die."

"Timesickness," JB diagnosed, his voice slightly smug. "You don't die from it, but like seasickness or airsickness, sometimes you want to."

The children are fighting to not do what JB insists they must, even as he dispatches them to various times.

The major differences are:

• time travel is a tachyon-based phenomenon, not quantum;
• JB is not a physicist;
• the children are not being used to carry out a series of thefts.