Answer
The answer is as follows. 
Katrielle and the Millionaires' Conspiracy walkthrough
Puzzle Location: Outside the Layton Detective Agency (Stachenscarfen)
The children playing at the park have all gone home.
Move one of the matches to spell out the reason why they went home.
You can move the matches by sliding them.
They cannot be rotated.
The children were playing happily at the park under a bright blue sky.
What could be the reason why they would go home?
All of the children that were at the park went home.
The reason why is time-related.
Hint: children don't usually play in parks at night.
The sky isn't bright blue at night.
How would the park look at night?
The answer is as follows.