Editing pre-existing sims relationships?

I moved a sim into another sim's household and I want to turn them from roommates to mother and child but I either get a "cannot edit pre-existing sim or the only options I get are "sister/brother, roommate or wife/husband." I changed their ages and the cheats are on both testingcheats true and cas.fulleditmode but it still doesn't give me the parent/child option.