React – the right way to pass form element state to sibling/parent elements?

• Suppose I have a React class P, which renders two child classes, C1 and C2.
• C1 contains an input field. I’ll refer to this input field as Foo.
• My goal is to let C2 react to changes in Foo.

I’ve come up with two solutions, but neither of them feels quite right.

First solution:

1. Assign P a state, state.input.
2. Create an onChange function in P, which takes in an event and sets state.input.
3. Pass this onChange to C1 as a props, and let C1 bind this.props.onChange to the onChange of Foo.

This works. Whenever the value of Foo changes, it triggers a setState in P, so P will have the input to pass to C2.

But it doesn’t feel quite right for the same reason: I’m setting the state of a parent element from a child element. This seems to betray the design principle of React: single-direction data flow.
Is this how I’m supposed to do it, or is there a more React-natural solution?

Second solution:

Just put Foo in P.

But is this a design principle I should follow when I structure my app—putting all form elements in the render of the highest-level class?

Like in my example, if I have a large rendering of C1, I really don’t want to put the whole render of C1 to render of P just because C1 has a form element.

How should I do it?