Objectively, Jiko is clearly the "better" restaurant when judged by traditional standards of quality, but that doesn't necessarily make it the correct choice in this situation. With children that young, Boma seems like the perfect place. Jiko deserves its title as a "signature." It's quiet, romantic and clearly geared toward adults. I don't mean to imply that kids are not welcome, but your clan will undoubtedly feel more comfortable at Boma, a buffet designed for families.
I love Jiko and consider it one of Disney's best restaurants, but Boma is also fantastic. It's easily my favorite buffet, offering better variety and food quality than all of its competitors. There are plenty of options for all palates, but Boma has enough African and African-inspired dishes to appeal to adventurous eaters. It's a giant leap beyond the standard prime rib and carved ham at Crystal Palace or 1900 Park Fare.