As far as I know, there is no ferry from Liberty Harbor to the Statue of Liberty. There is a water taxi that will take you from LH to the Wall St area right in the park, but it is $$, especially for the 5 of you. If you walk 3 blocks, you can get the PATH into the World Trade Center (911 Memorial) or to the Times Square area (33rd St.). The cost of this per person was $2.50 (2 summers ago) but the water taxi was $10 pp. If you take the PATH to Times Square, you can pick up one of the Gray Line buses right there. They are one of the ones that do the on/off tours. It will take you to Battery Park and then you catch a ferry to the statue and/or Ellis Island.
Just to let you know, August is the hottest time of year to be visiting these cities. It is not unusual for several days of 100+ temps.
In DC, you really don't need to do the trolley thing. Purchase an all-day pass on the Metro. You can ride the bus from Cherry Hill and hop on the METRO. Or drive to one of the park/ride facilities OUTSIDE the city and ride in. The METRO goes to ALL the sites you are wanting to see, but you may be doing some walking. There are 2 METRO stops in/around the MALL. The Smithsonian museums, the Washington Monument, Capital, White House, Lincoln Memorial, Vietnam, WW2, and the others are RIGHT THERE. Arington National Cemetery is on another stop as is the National zoo. An all day pass is about $14 per person. The youngest may be able to ride with an adult and not cost a thing. Our daughters, 10 and 7 really enjoy riding the Metro and that is something they don't get to do at home. The enjoyed Air and Space and the Native American museums along with the Natural History museum. We bought food from a vending cart and had lunch on the lawn on the mall. (We went last June). They were also really interested in the changing of the guard at the cemetery. (Be sure and explain to them about the importance of being quiet and respectful prior to getting there.)
The National Aquarium is very good in Baltimore. My girls also enjoyed using the water taxi in the Inner Harbor to get to some of the other sites. Fells Point is interesting. (Did Baltimore in July last year).
We have been to Assateague, we have done the Naval Base, Colonial Williamsburg, but I have never done Macinac Island nor Greenfield Village. It is something they have or will study in school, so it is at least informative and educational.
Ocean City does have a great boardwalk, but it is usually crowded and VERY, VERY commercial. For a little different atmosphere, check out Rehobeth Beach, DE, which is about 30 miles north of OC and has a great little boardwalk and is in a very artsy community. Not as much commercialized as OC. Great places to eat there.
Hope this helps.
