They are three very different atmospheres, all with good food.
Jiko is a signature restaurant and therefore not as kid-friendly. If you feel all your girls are mature enough to enjoy a long meal with no extra entertainment, then I would recommend Jiko since the food is the best of the three.
That said, you can't really compare the three places since they're not meant to compete on the same level.
Sanaa is a casual table service with Indian-inspired cuisine. This would be my pick if you don't think a long, fancy-ish meal is a good idea. The atmosphere here is very relaxed, a bit intimate, and they have great views if the savannah, so you may get to see animals while you eat.
Boma is a loud and chaotic African-inspired buffet. It's a busy place with a lot of hustle and bustle, so while the food is good, it's not what I'd call a relaxing meal.