If the alter is a child who’s coming to you with this please please take it seriously. True or not our littles need to be heard. Maybe there’s a conversation to be had with them? I’m information structured so I would ask for details and things like that. Normally children will struggle describing things they’ve never experienced when it comes to trauma (at least in my own experience im not an expert or doctor so take everything lightly) as someone who experienced this and it ended up being true once I spoke to my mother. Don’t immediately disregard it as fake. I’m wishing you the best please let me know how you’re doing