What's the Name of That Book??? discussion

It’s not new technology it’s woodsy in the war. The creatures they are fighting might also be other kids but they have a different name for them. At the end of the book he and all of the other children (all the people fighting the war are children) are allowed to go back into their own time (they all enter this world at different times, some have been there for years others for months. When he goes back into his time he meets one of his friends from the war reality but the friend is an adult because when he went into the sewer was before the protagonist was born. (Example because I can’t remember: maybe 1 kid went into the sewer and had to start fighting the war in 1930 when that kid was 10- so while he was fighting he remained at age 10 the whole time. Another kid at age 15 went into the sewer/ alternate reality war in 1950 (protagonist). So after the war when they all went back to their own time they felt like years had passed in the alternate reality but really no time had passed. So the kid from 1930 was 10 and his friend he met in the alternate reality wasn’t even born yet because it was like he went back in time. But after living for 20 years in reality it was 1950 and he knew his friend would know him.

This book was read out loud to me in 2013 ish in my 6th grade English class. I feel like the time period of the book was 1930s- 1950s but was likely written in 90s-2010.

Not many books on this list yet, but it's relevant: Sewer Scene - https://www.goodreads.com/list/show/1...