I think eating in the dining car will be a treat for kids of any age.
A few other things to look for:
While still in the mid-Atlantic, you’ll get the Philly skyline and the zoo on the left. Also, on the right side at Wilmington, there is a mural of a whale on the side of an office building—the kids can have a contest seeing who can spot it first.
At Washington, it’s fun to watch the engine change, especially if it’s the first time you’ve seen it.
Leaving Washington, you get the Washington Monument and Jefferson Memorial on the right.
I don’t know how far you’re going in Florida, but there is a nice view of Winter Park (the stop before Orlando) on the left.
I’m from NJ, too, and have taken the Star to Winter Park several times and enjoyed the trip.
I think NJ to Florida (even if you go all the way to Miami) is a perfect length for a first long-distance trip.
Hope you all have a wonderful time, and please let us know how it goes.