Kids are a bit young to really enjoy the great wall. The hike from Jinshanling to Simatai is breathtaking. You can do the shorter trip with them, or leave them at home for a day. The 1/2 day walk between the two entrances is simply spectacular.

Suggestions:

Zoo (pandas) & Summer Palace (could be a lazy day each, or both in one day)
Tianenmen Square / Forbidden City
Back Lakes (book a dinner boat)
Temple of Heaven / Antiques Market (close to each other)
Hutong visit

Consider a day trip to Xi'an to see the warriors. Flights are cheap on ctrip.com