I am in the process of creating an interactive adult murder mystery dinner theatre for our company (performance in October).  In my opinion, the best bet is to create your own, but you need to know exactly how interactive you want it to be.  The more interactive, the more difficult for younger children, as it tends to be all about having a solid character and improvising rather than memorization.  Preconcieved online scripts (mostly for adults, as far as I can tell) seem to be running anywhere from $45 to $100 for a single performance.  A lot (NOT ALL) of those are allowed to be changed, edited, and modified at the discression of the producer.  One of these, while not useable in content, may be what you need to create the structure of your own, original show.  It wouldn't necessarily even have to be a murder,  involving kids it could just be a crime.  The Encylopedia Brown series of childrens books is all about an adolescent who solves neighborhood crimes - these might be great plot inspiration.  If you are looking to do a non-interactive show, basically a mystery presented as a play while dinner is served...I got nothin', unless you write your own. 

Thanks for the suggestions and advice.  I was actually able to find one that met my criteria written by none other than A.A. Milne.  It's based on his book The Red House Mystery.  I've never heard of the book, but can only imagine Pooh, Tigger, and Eeyore being involved somehow. 

I'm not a huge fan myself, but it might be worthwhile checking out "Mysteries By Moushey" (Google it).  Our local theatre does one of these every summer and they tend to go over well.  I'm not sure if she has any plays for younger groups, but it never hurts to look.

Design for Murder