The children in the photo were imprisoned due to their ethnicity - Russian. They were victims of ethnic cleansing.

More info:

https://en.wikipedia.org/wiki/East_Karelian_concentration_camps

https://en.wikipedia.org/wiki/War-responsibility_trials_in_Finland

https://www.histclo.com/essay/war/ww2/cou/fin/cw/cw-kz.html

https://forum.axishistory.com/viewtopic.php?t=176121

https://catflag.wordpress.com/2018/06/10/finland-in-wwii-the-democratic-axis-power/