imo:
bad idea.
i hate gettin tattooed at convention.
besides, you cant tel me the tattooist can give it his or her all with all the stresses and time pressure of a convention, (dont care which artists deny it, you know your fibbin :P )
i hear of conventions closing while people are still being tattooed etc. and the tattooist hav ing to stop etc.
why bother and have the hassle?
go for the studio thing.. although im not sure many artists will tattoo a 13 yr old.