Because of flight changes, our family trip to Barcelona (with two kids, 13 and 9) have about 7 hours between flights in Berlin Tegel, should we sit at Tegel Airport or take a quick look at the city? It's first time we in Berlin.
I would take advantage of getting out into the city if you possibly can, and you feel that you've got time to run through customs and find your next flight in time.
You can certainly get into Berlin; Tegel is actually inside city limits, and only about a 15-20 minute cab ride to many points of interest. Exactly when are the 7 hours? That way, people can make specific recommendations (no point in recommending you see things that will be closed during your layover).
Thanks for your comments, We will arrive at Tegel airport around 7:30AM, and our next flight will be at 3:00PM.
I would take the TXL bus to Unter den Linden (maybe 20-30 minutes), and get off near the Brandenburg Gate. From there, you can walk or take the U-55 to the Bundestag, where if you have reservations, you can walk to into the dome. If not, you can admire the building's exterior. Or you can go the opposite direction, towards Potsdamer Platz, stopping to look at the Memorial to the Murdered Jews of Europe, with its free museum within the blocks.
From there, you can walk east towards Alexanderplatz, passing Museuminsel and the Berliner Dom. Or you can spend some time in the Tiergarten, particularly if it's a nice day. The TXL bus may be picked up anywhere on Unter den Linden. The terminus is Alexanderplatz. Enjoy!