We have a five hour lay over in Berlin and are considering leaving the airport to see the Brandenburg Gate. By taking public transportation, not a taxi, how would you recommend our family of 6 (4 kids, 2 adults) get there and do you think we'd have enough time (5hours)? If we don't have enough time by using public transportation, would we have time if we took a taxi? And if yes, how much would this cost each way and how long would it take, roughly? Thanks!

While I do no not personally know Berlin (others will weigh in),
it looks like there is a bus from Tegel Airport (will that be YOUR airport?) to the Brandenburger Tor stop at least 5X per hour and lasting 29 minutes ONE WAY. Individual ticket prices (one-way)look to be 2,40 Euro per person with a rate of 15,50 Euro (one-way) for "small groups" (don't know what that entails either...?
See:

http://reiseauskunft.bahn.de/bin/que...fasScrollDir=1

The real question is whether you TRULY have 5 hours to do this in...ie: will you have to check in for a connecting flight and go through security again, etc....

If you connect at Berlin Tegel airport, five hours should be enough for a quick trip into the city center.
But check your airline's regulations how soon you have to be back.

The regular city bus (line TXL) will take you directly from Tegel airport to one block away from Brandenburg Gate. That line is sometimes stuck in a bit of traffic so you should calculate 40mins each way (usually it's more like 30mins).
What tickets you need will depend on the ages of your kids. But in general, the whole "round trip" will not cost more than €20.

A taxi will be a bit quicker and will cost around €25 each way. But you will have to find a minivan cab in the lines, as a regular taxi can only take 4 people. If you travel with kids under 12 or under a certain height, they must travel in booster seats - of which not each taxi may carry more than one or two.

There is something called "Small Group Ticket" for 15.50 for all day http://tinyurl.com/l2dha2f. TXL bus stops near Brandenburger Tor S/U bahn station

We actually made it with no problems and plenty of time to spare! Ended up catching the TXL bus line and walked through Brandenburg Gate. Also saw the US Embassy; couldn't go in, but saw it.