When my own kids were younger they asked me to devise a murder mystery game for them, as they were aware of this type of party activity, for adults and older children. I game up with the following mystery game party ideas, which allowed for all to participate and provided a lot of amusement.
A simple murder mystery game, for a kid’s party, would involve hiding clues around a house (or garden) and have the kids find them and collectively solve the murder. The crime need not be a murder, i.e. it could be a robbery, but the types of clues found would be similar in that they eliminate a suspect.
In its simplest form a list of 12 suspects would be written on a board for all to see, the suspects could be any names, e.g. Mr. Smith, Mrs. Jones etc. The party quests then look for notes of paper (that you the hostess has hidden) which have a name written on them. There needs to be at least 11 notes, one for each suspect being eliminated, but there could be more as duplicate notes could be made. The important point being that the culprit is not on any note and once the kids have found 11 unique names on the notes they will be able to deduce the murderer (or robber).
If your guests have problems finding the notes, after a certain time, you can give them hints. The idea is the whole group solves the mystery together.
If you want to introduce an element of friendly ‘competition’ then have the party guests split into small teams (of detectives) and each team competes against the other. For this version you will need a lot of notes (with duplicates) scattered around as the kids will ‘destroy the evidence’ when they find a given ‘note.’ to try and hinder their competition. The team that makes the correct deduction (solves the crime) first wins. For this version of the game you will need to give each team their own list of suspects so they can discuss and eliminate ‘in private’.
A more complicated version of this murder mystery party game (suitable for older kids) would be to have your guests find two pieces of information (notes) to eliminate a given suspect. The set up is the same (with 12 suspects being written on a board or given out to the teams) but the clues are in two parts. The first part (note) is something like “A person that owns a Parrot is innocent” and the second part (note) is “Mr. Smith owns a parrot”. In this case Mr. Smith is eliminated when the two notes are found and matched. If, however, there is no matching note for “Mr. Grey owns a Cat” then Mr. Grey cannot be eliminated and he would be the culprit after elimination of the 11 other suspects. As with the simpler game this can be played by the whole party or with teams of ‘Detectives’. If you want to make it even more complicated you can have three or more pieces of information. For example “The person who owns a Dog and Cat is innocent” will match (and eliminate) with the two other notes: “Mr. Smith owns a Dog” and “Mr. Smith owns a Cat.”
You can use your imagination with the clues, so as to fit a theme of your party. For example “The Princess that has 7 Facebook accounts is innocent” matches with “Princess Mary has 7 Facebook accounts”. The key to the game is the finding, and optionally matching, of clues followed by elimination of the suspects to deduce the culprit.