According to the Citi Field Amenities Map on the Mets' website, "Mr. Met's Kids Kitchen" is located in section 141, next to the Mr. Softee concession. Best photo I have that might be able to help is this:
(Photo taken September 28, 2013. © Gary Dunaier. Link to upload on Flickr.com: here.)
Obviously, the photo only shows one concession. But if memory serves me - I saw the concession, but I wasn't really paying attention to it - the "Mr. Met's Kids Kitchen" was where, in the photo, the reflection of the skyline from the Shea scoreboard is.
Hopefully someone else will be able to provide more accurate info.