Possibility to directly access children of a window?

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

Hey there,
I am wondering if there is any possibility to access children of a window directly. It would be very convenient to have this as it would make updating/removing child widgets a lot easier.
If that is not possible: what is the reason to not exposing the children?

Thank you in advance!