It's never too early to start Christmas shopping... From the Science Museum:
Science Museum Fingerprint Kit
Find out who's been raiding the biscuit tin. This detective's kit contains all you need to find and record fingerprints, including dusting powder, fingerprint record cards, stamp pad, magnifying glass, stickers and a brush. Supplied in a handy case with instructions and fun facts.
Science Museum Go Detective
Become an amateur detective. Learn how to search for clues and solve crimes using scientific methods. Includes over 65 fun activities to help budding super sleuths make use of the smallest details and clues to come to the right conclusions.
(This isn't listed on the Science Museum site currently but is available at other shops eg summertoys.co.uk and I saw it in W H Smith's today.)