I noticed a post with the same question before
How to change size/shape of Holographic button? - Questions - Babylon.js
Unfortunately, this code doesn’t run on typescript
I got this error
Not checking the source, but a child is a Node. You can have children of meshes which areof type Light, Camera, TranformNodes, AbstractMesh, Mesh, InstancedMesh, & Node.
You can either cast the [1]th element as an AbstractMesh, or use getChildMeshes() instead of getChildren(). That hardcoded [1], looks like crap code, which could break in the future though, or even now, since it looks like [1] is undefined