For posterity, here’s what was going on. There was a field in the related (child record) model that was a formula, and as such was read-only. The intended functionality was to set the Contact field based on the cloned child records. Once we swapped the formula field for the Contact field, it worked like a charm. 

This reply was created from a merged topic originally titled clone two tabs that have parent child relation. we have the following error, while trying to clone two tabs that has parent child relationship
"Attempted to add a campaign member where either the member id ‘null’ or the campaign id ### ‘null’