Deduplicate the warning about functions not being valid children

Same as https://github.com/facebook/react/issues/11081, but for this one:

<img width="476" alt="screen shot 2017-10-04 at 16 35 07" src="https://user-images.githubusercontent.com/810438/31184704-0d705000-a922-11e7-852e-5890962ff1e5.png">
