I vaguely remember the manga but it's about some children getting sent into another dimension and have to fight the enemy(alien) and after they kill it they got sent back into their world. to fight that the children got superpower. The main character is little female that has a problem with her family that she feels okay to sacrifice them but she is searching for her mother (if i'm not mistaken). After the first arc, the game watcher joins them in fighting another enemy. This manga has psychological genre and the art is quiet plain and it is not a gore manga.

I think it's sekai oni

Ah yeah it is sekai oni. Thx for your reply!