Akvaryum , a set on Flickr.
When it rains, it rains. The water dropping or falling down in Istanbul means it is not easy to find any activity with children unless you are ready to die by shopping (for shopaholics is this city HEAVEN).
So feeling like a doing something else and not sure what, take your family to Akvaryum (Florya). Parking lot is a comfortable to park at, but we were surprised that we had to pay (ok, it is 5TL, but the entrance fee is not cheap).
I would mention that the visitor´s space is really great one. You can sit and watch and go into bubbles and observe, you can find old ships (e.g. Liberty and older ones).
So no need to do more. GO, sit in the Coffee shop (I took the most delicious burek I ever had in TR there).