Admylin
Hmm, your DC's are a good bit older than mine, so they might not be so impressed by the zoo. There is a fabulous playarea, but again I do not know how older children would find it. I have very happy memories of the playpark, I used to let DD run wild while feeding DS. There is a cafe right next to the playpark so I would settle in with a coffee and a book.
Otherwise, that area is not far from List, which is a lovely Stadtteil. Lister Meile is a very nice shopping street, look out for Bagel Brothers and The English Shop there.
The Mövenpick cafe in the town centre is great for breakfast, Cafe Kröpke, next to the Kröpke Uhr.
Right opposite the Kröpke Uhr is a great ice cream parlour, they opened just before we left.
Moving down towards the Rathaus, have a look at the Markthalle. It is an indoor market, some fruit and veg and dozens of stalls selling delicious fast meals. Great for a quick snack, cheap and cheerful pasta dishes. Really fab.
Maschsee is lovely for a day out, DH was in Hannover recently and jogged around it. You have just missed the Maschseefest, it was on last week. It is a great festival, with bands playing all around the lake.
What else? A great day out is a trip to the Steinhuder Meer, about 40 min from Hannover, iirr. You can hire a paddle boat for an hour and have a coffee in one of the little cafes. Don't forget to buy some smoked eel on your way home :-)
If I think of anything else, I will be back.