If you have spent the morning around Alexander Platz I can recommend a lunchtime pit stop at the cafe on the top floor of Galeria Kaufhof. They have a great buffet selection of salad, cold dishes such as open sandwiches and hot dishes. You load up your plate and pay by weight. The desserts are great as well, we enjoyed the custard and strawberries!
There is loads of seating at different levels and lots of high chairs. The toilet are on this floor but not in the cafe. It is also important to note that you have to return your plates etc on their tray at a spot in the front of the cafe.