I have a 3rd grader, and we attended the program last weekend in Baltimore. My dd's passion is science, so she had the time of her life. They spent half the day on a boat on the Chesapeake where they did water quality testing, navigation, and had the opportunity to drop the nets and investigate what they caught. The other half of the day was spent dissecting oysters, identifying plankton in local water samples, and calculating and comparing the square footage on a map of the port of Baltimore from the 1700's to a current map.
I definitely agree that the 5-7th grade options seem much more interesting, and we're anxious to get there. However, we felt that this activity was run in a way that was appropriate for the kids involved. Most of the kids there that day were in fourth grade. In our group there was one other third grader and one second grader.
I am familiar with the aquarium the program you are considering will be held at, and I have also wondered how involved it will be. We have family in that area and have been there a few times and felt we have seen all there was to see there.
Maybe someone with more experience at these programs will chime in. I am hoping they're all as wonderful as the one we attended last weekend.