Royalty & Legacy :Promissing Noble Inheritance to family member

Hi guy's,

My sims is in the noble career as a Margravine.  She has the task to Promise Noble Inheritance Title to family member.  I wan't to give it to her first born son who is a child but I can't seem to find the interaction anywhere.  It's keeping me from getting promoted...anyone as any idea how to do it???

Thanks in advance

Sylune