It will probably be difficult. You might just want to read them some Encyclopedia Brown books.
My company Mystery Factory has kids party instructions. 'Fossil Furor at the Mudstone Museum' comes with an audio clip of the crime (stealing a fossil). You can listen to a sample of the clip at the Mystery Factory website but to get the whole clip you have to buy the mystery.
It's a game that works in conjunction with the clip to solve -whodunnit.It runs as a simple crime scene investigation and clue matching game. You can probably tell from the sample if it's too advanced but if they are gifted it might work.