Hi there,
I am planning to spend 14 days visiting East Cost starting from Washington DC with my family. I have 3 children (15, 11 and 7)
I will be driving all through.
any hint where to go what to do?
Washington DC, NY, Philadelphia to be visited.
any hints?

You really need to get a guidebook.

A good starting point would be the "Destinations" tab at the top of this page. Plan to spend 3-4 days in DC, 1-2 in Philadelphia and at least 4 in Manhattan.

What are your interests? What time of year will you be visiting?

Stop by Ocean City, MD then drive S to Norfolk, VA. Head S to the Outer Banks and on to Beaufort, NC. Then head S to Pawley's Island, SC and Charleston, SC. Spend 3 nights in Charleston and head S to Savannah. After a few days visiting the HD in Savannah head S to Sea Island, GA and then on to Amelia Island, FL. Enjoy your trip.

Driving in any of those cities is a nonsense. There is never any place to park and garages are expensive ($30 to $40 per night) for a car that you can't use during the day (or you will be constantly putting into different garages at $12 a pop). In all of those places you will have to pay for public transit - so having a car too doesn;t make sense - unless that is the only way to get to DC and back from NY or vice versa.

Do you want to head north from DC or south?

As sf7307 stated, those three destinations will take up your 14 days. One transportation option that may be fun is to take the train between destinations. Public transportation is a must in all three cities. For DC accommodations you may want to stay in in Virginia as hotels may be more reasonably priced. However, that all depends on what time of year you are going.

I was just in DC. Obviously, you will want to see the monuments and a few of the Smithsonian museums. The Air & Space is fun. The Newseum is also fun but it is one of the few museums that have an entrance fee. I found the Fodor's guide book to be very helpful.

You don;t say when you are coming and that makes a big difference in terms of weather, prices and what to see/do. I assume you are going to be 5 people total - which means a suite or 2 rooms - you can;t put 5 in one room.

If you provide info on when you're coming, budget and interests - people can make specific recos on where to stay and what to do. For NYC esp - and also some times in DC - getting a hotel far in advance gives you the best chance at a good deal.

But it's hard to help you without more info.