API to assign item to a new child organization

Hi oracle

I see blogs to use the below API to assign an item to a new child organization.

We have already existing master inventory organization and are introducing a new inventory org.

Need of the hour is to assign all items in Master to the child org.  Can I get this done using the API below.. But this is throwing master child conflict errors in script, whereas if I try the same for an item using Front end Inventort form , it does it with success.

Why is this discrepancy between API and form ?

How can I handle this in the samw way as the front end form handles an item organization assignment