We ran this for a seventh birthday and it was the first party where I did not once have to announce the next activity. They were busy from the door.
The crime was that the birthday cake had been stolen. Six suspects, all family members, each with an alibi pinned to the wall and one contradiction hidden in the details.
They found it. It took forty minutes and one very loud argument in front of the clue wall, and then the cake was discovered in the garden shed and everybody ate it as evidence.
Tip
Make the contradiction obvious enough that a six-year-old finds it. My first version was far too subtle and I had to feed them hints, which spoils the whole feeling of having solved it. And hide the cake somewhere they can genuinely reach.