No question, with kids that age I'd choose Boma.
Boma is a buffet, with a section of kids foods and a huge variety of choices for everyone. Since it is a buffet, you can control the length of your meal. It's a bustling place, fairly noisy, so if the kids are getting a bit 'enthusiastic' and making a bit of noise, it's going to be lost in the general din.
Jiko is Signature Dining. It's elegant, quieter and definitely more upscale, and the meal may take a couple of hours. They do have a kids menu, and you certainly won't be made to feel unwelcome if you bring the little ones, but it's a 'sit still and be quiet' kind of place, and the kids may not enjoy that a whole lot.