How to get offsetTop of a React Child with TypeScript

How to get a React children offsetTop using Typescript ?

Here my component:

export default class FadeIn extends Component {
  private onScroll = () => {
    React.Children.forEach(this.props.children, child => {
      // Get the child's offsetTop here and do stuff with
    })
  }

  public componentDidMount() {
    window.addEventListener('scroll', this.onScroll)
  }

  public componentWillUnmount() {
    window.removeEventListener('scroll', this.onScroll)
  }

  public render() {
    return this.props.children
  }
}

I already tried to:

• use ReactDOM.findDOMNode(child) but I get Argument of type 'ReactChild' is not assignable to parameter of type 'ReactInstance'. Type 'string' is not assignable to type 'ReactInstance'
• use getBoundingClientRect but I get Property 'getBoundingClientRect' does not exist on type 'ReactChild'. Property 'getBoundingClientRect' does not exist on type 'string'
• cast child to ReactElement<any>

Any idea ?