Seems the Animal viewing is the draw for children at Sanaa. I'd suggest attending lunch or if you're going in summer when the days are longer try an early dinner.
Honestly, depending on what you think you'll order at Sanaa, price for the family may not be less or much less vs Boma. We were two adults and had a bowl of soup, bread service, two entrees (shrimp and fish selections), pot of tea and dessert sampler. Dinner at Boma would have been $33 each. I didn't see the final bill, but I bet it was over $66.
I loved our meal at Sanaa and its on our do over list. I'm also a fan of Boma dinner. I'm not sure I'd choose Sanaa as the "budget" option vs Boma. Two great places with different dining experiences.