I figured out a solution for this issue myself. This is how i accessed the parent.children of children nodes.
foreach (var parent in Nodes)
{
if (parent.IsChecked == true)
{
foreach (var item in parent.Children)
{
if (item.IsChecked == true)
{
}
else
{
}
foreach(var vtemp in item.Children)
{
foreach (var vtemp1 in vtemp.Children)
{
}
}
}
else
{
}
}