Hello,
go to CSM Settings > Customize Start Menu and move/delete the Programs item (on the top of list) to the right.
Be aware the user can still override this themselves as CSM settings are per-user. To block this use Group Policies on the computer, have a look at this:
viewtopic.php?t=269
Probably a good idea to remove other items too, like Run and Settings. So the only things on the menu will be the pinned and recent programs.
However, also know the user can open the standard Win10 menu by pressing Shift+Win key. You can override this in Controls tab. Maybe just set Windows-key opens nothing, left-click opens nothing, Shift+Win, Shift+Click the same etc.
Don't underestimate the computer skills of children, if there is a way around they will find it eventually
