It is best to buy tickets online and in advance when visiting the Intrepid Museum. It makes getting through the entrance faster.
It can be very hot on top of the aircraft carrier, so hats are a good idea. On the other end of the spectrum, the submarine museum is a tour of an actual submarine. The entire tour takes place in a tight enclosed space.
There is a lot of walking at the Intrepid Museum, so the best time of day would be when you and your family have the most energy.
The museum wasn't too crowded in August, arguably at the height of tourist season in New York, so it seems like anytime would be good to visit.
There is enough to do and see at the museum, you don't need to spend the extra money on additional activities such as the G-force simulator or the 4D experience.
If your family is interested in things that fly, sail on the sea or dive below it, then the Intrepid Museum with kids is a must see. It boasts many different, amazing things in just one location. Begin by learning about the inner workings of an aircraft carrier. Then, check out the variety of planes, from across the decades, up close. Afterward, squeeze your way through the tight quarters of a submarine. After a fun, but long day, head to Riverside Park and enjoy dining al fresco at Ellington in the Park. Here you can relax and grab a drink while the kids play in the park. All in all, a wonderful day in New York with kids.