Justice Light's lost children, remembered only through the puzzles they loved as much as he does.
Justice Light mentions his children exactly once, in a single devastating aside: he enjoys the Sluggalos because their slang is a never-ending supply of puzzles to solve, and Justice Light loves puzzles. 'So had his children.' The past tense is the whole of what the reader is given.
The detail resonates against the rest of his confession — the Ice Elves also loved puzzles, and played in the snow like children, which is part of why killing them broke him. Nothing else about his children is said: not their names, not their number, not their fate beyond the tense of the verb.
The quiet loss underlying Justice Light's affection for puzzles and for gentle creatures.