How do I set "ischecked" for nested toggle buttons (WPF, C#)

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C#

XAML

MVVM

Binding

Hi everyone,
I am fairly new with using WPF and I jumped right in and used the MVVM pattern. I have a project where I need to set IsChecked = true for nested toggle button i.e. toggle button inside a toggle button.
Background: I am doing an app to display an organizational structure so at run time the toggle buttons get created. I used toggle buttons because they show the hierarchy nicely plus I needed the state - when the button is checked, change color and add to list somewhere (ignore this part). This works perfectly when I click on the parent node but what I want is for the child nodes within that parent node to be changed as well.

N.B. there is only one togglebutton in my XAML

P.S. The "base idea" for this project is taken from Hierarchy Application - SharePoint[^]

What I have tried:

<togglebutton x:name="btnNodes" template="{StaticResource ctpDetailButton}" command="{Binding SelectedCommand}" ischecked="{Binding IsSelected,Mode=TwoWay,UpdateSourceTrigger=Explicit}">
ViewModel: I have a property "IsSelected" and I set it to true on PropertyChanged" (exactly as done in the link. Then I loop through the children nodes to set isSelected to true. This works but it does not change the background color for the toggle buttons inside the "parent" toggle button.