I hate to stoke the fire, but I think recommending a 6D or 5D3 to the OP, or telling him to photograph a "moving" subject instead, is a terrible idea. Trying to argue that the 7D2 can only photograph sports/action, and is not suited for still subjects, is just, well, silly. It's a camera. Granted, it's a camera with a certain design intent, but it's not like we're comparing a Chevy Nova to a track-ready F1 race car.
The 7D2 is a mid-level pro-sumer body, designed to handle a wide array of subject matter and shooting conditions. It should certainly be able to focus on a child in a kitchen in relatively medium/low lighting conditions. Now, that said, low-light conditions are certainly where AF systems struggle the most.
I will finish this post off by saying this to the OP: After thinking about your posts/story more, the more I think your 7DI (which, based on your accounts, focused more accurately and repeatedly in all lighting conditions) is the likely outlier here than your 7D2. I use a 6D and 5D3 in my event work, and I constantly find myself in demanding situations where there is very little available light. I usually start events at ISO 3,200 and often push to 8,000 as required. Missed focus comes with the territory, and getting those tack sharp results in such conditions is a real chore. So, the fact that you think your 7DI is spot on, time after time in every single scenario, tells me you may have gotten incredibly lucky with that body. All photographers struggle with adapting/modifying their AF techniques as lighting conditions change. To expect repeatable/consistent AF in low-light is simply not realistic, and is not what you should expect with current technology. It's a sensor; it can only do so much.
