Choose itineraries where there's not too much driving involved (i.e. a lot of parks to visit) especially if you have young children. It's also a good idea to opt for lodges with swimming pools to fill up the time in between game drives if your children are younger. Find out if there are child-friendly programs on site which will occupy the children if you are out on a long game drive.
Note that some safari lodges won't accommodate children under 6. Avoid long game-drives or drives between lodges in safari parks if you have younger children since they won't be able to get out for a bathroom stop due to the wild animals.
Recommended Family Safaris in Tanzania:
African Portfolio offers a 9 day family safari in northern Tanzania.
A two week Tanzanian family safari. (Recommended for ages 6 and over) includes the Serengeti, Ngorongoro Crater, Tarangire and more.
2 Week Family Safari from Thomson safaris also includes Zanzibar, Olduvai, Ngorongoro Crater, Serengeti and more. Thomson safaris specialize in Tanzania and have an excellent reputation.
Serengeti Family Safari from Wildland offers a good safari itinerary for those with teens that includes all the major northern parks in Tanzania including opportunities to mingle with local tribes, a visit to a school, and camping in the bush.