Created Issue: Update cascading drop down field by code [28641]

I'd like to update by code several items in a list with drop down field.
My code:
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item["fieldParent"] = valueParent;
item["fieldChild"] = valueChild;
item["fieldChildChild"] = valueChildChild;
item.SystemUpdate();
list.Update();
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It works for parent but not for child fields; text is updated in the display column (allitems.aspx) but in EditForm, the selected value in combo box is incorrect (first item displyed by default)

How can I do to update the 3 fields with consistent data?

Thanks.