OP, I think whether it's a "must see" depends on your 5-year-old. Our pre-schooler absolutely loves Remy (regularly requests a piece of cheese and one strawberry "just like Remy" at the beginning of the movie.) He loves Remy and is thrilled to see him at Chef's. On our most recent trip, Remy's "handler" was particularly great - spending lots of time initiating conversation between him and Remy. And our two older boys (pre-teens) enjoyed it too.
But for kids not excited about Remy or not familiar with the movie, I'm not sure it would be as much of a priority.