We went in February half term a few years ago, it was really cold (think ice floating down the river), so i would factor in lots of cafe stops! We went to a chocolate shop with delicious hot chocolate upstairs called Fassbender and Rausch, looked at the ostside galerie, went up the fernsehturm (the dds, 6 and 4 were not at all interested in this, despite us rebranding it as rapunzel's tower..., yours might enjoy it more, being older). The reichstag is well worth a visit (you can book in advance online, or queue up and go in, but make sure you take your passports).
Guidebooks recommend taking the 100 bus, which apparently does the same route as the tourist bus, but for the cost of a single ticket.
You can get group tickets for public transport (which i think worked out cheaper for me, dp and dd1, dd2 was too young to pay) so will certainly work out cheaper for four of you.
Oh the holocaust memorial was also good (if that's the right word for something so awful).
And as a german teacher, can i commend you for taking your ds to germany? I wish my year 10s would go, they might start to see the point in learning the language then!
We loved berlin so much we went back the following summer (ludicrously hot, we spent a lot of time finding bits of river to swim in, we had a great time).