Suggestion: New parent-child relationship type of "Lived Together"
Since there's a couple relationship type called "Lived Together" when you know two people were a couple and you know when they lived together, but not with certainty when or if they ever married, I'd like to propose a similar relationship type for parents and children.
This would be helpful, because some records indicate a child and one or two adults are part of the same household, and implies that the adult or adults are responsible for the care of the child, but you're not exactly sure if or how they're related. This is very common in older censuses, for example.
A "Lived Together" relationship type would allow users to clearly record when and where a child was first observed to be part of that adult's household, and it would be an obvious signal to other users that additional research might be required.
I think that would be a better solution in many cases than the "Foster" or "Guardianship" relationship types, since those can imply an official or formal relationship between children and adults that there's often no record of.