In your parent, you can create a function like addTodoItem which will do the required setState and then pass that function as props to the child component.
var Todos = React.createClass({
...
addTodoItem: function(todoItem) {
this.setState(({ todos }) => ({ todos: { ...todos, todoItem } }));
},
render: function() {
...
return <div>
<h3>Todo(s)</h3>
{todos}
<TodoForm addTodoItem={this.addTodoItem} />
</div>
}
});
var TodoForm = React.createClass({
handleClick: function(e) {
e.preventDefault();
this.props.addTodoItem(this.state.todoInput);
this.setState({todoInput: ""});
},
...
});
You can invoke addTodoItem in TodoForm’s handleClick. This will do a setState on the parent which will render the newly added todo item. Hope you get the idea.