Sometimes the grandparents, or relatives of a person, are known but at the time of recording the parent is unknown or uncertain. For example, at the time of its first census, a child was visiting its grandparents and was not with its family on the next census. Often, in these cases, the child is noted with their surname so could be the legitimate child of a son or the illegitimate one of an unmarried daughter. There will be numerous other examples.
There is no convention but some simply put in a dummy record as a parent for the child with the same surname as the Grandparents. For example if Baby Jones is noted with James and Martha Smith, it could be recorded thus:
James Smith + Martha (Smith) | Unknown Smith + Unknown Jones Female Male | Baby Jones
Another example if Baby Smith is noted with James Smith and Spouse it could be recorded thus:
James Smith + Martha (Smith) | Unknown Smith Male | Baby Smith
If one is confident that James and Martha's own offspring are all known but the identity of Baby's parent is in doubt, possibly enter it as Known Smith and create a local note against the possible parents or a family note against James and Martha detailing your findings.
If Baby's parents are later found, you can change the dummy details if the parent is totally new to your records, or either move Baby to the real parent and delete the dummy, or merge the dummy with it.
If the child is with a relative, you may need to create further dummy records to build up the relationship.