Fenris and Hrefna were mutually toxic lovers that everyone thought would end up together when he was young. After expecting the power and luxury of being the chief’s wife Hrefna was pissed off when she basically got thrown away after he grew cold to her. The bastard children are not even legally his as well, but to deny them outright would make it seem like he was trying to suppress their potential future claims to the throne and basically make them figueheads to justify rebellion. Ultimately they are a way for Hrefna to have a little power over Fenris still even though they broke up so long ago, it means he can’t outright dismiss her without causing trouble for the kingdom.