navlink activestyle not working

render() {
    return (
      <div>
        <h5>
          <NavLink
            to="/child-a"
            activeStyle={{ color:'red' }}
            exact
          >child-a</NavLink>
        </h5>
        <h5>
          <NavLink
            to="/child-b"
            activeStyle={{ color:'red' }}
            exact
          >child-b</NavLink>
        </h5>
        <div>
          <div><h2>Hello</h2></div>
          {this.props.children}
        </div>
      </div>
    );
  }