React.children only expected to receive a single React element child.

When used in conjunction with the material-ui library (http://www.material-ui.com/#/), the above error is generated in response to passing a single function as the `children` for any of the `Motion` components. If instead an array containing a single function is passed, then the following error is generated:

```
Failed prop type: Invalid prop `children` of type `array` supplied to `Motion`, expected `function
```