You would be considered a preferred option for these kids (assuming you meet all of the fostering criteria) if their dad is not suitable. If dad is stable then there is a very real chance that they will be placed with him over you. The courts feel that a child is better off with a biological parent if possible. I think it would depend on the type of relationship you have with the kids and how often you have seen them. If the children have been living with you for an extended period and they don’t know their dad the courts will take that into account but it doesn’t mean that you will automatically get custody. Like the others have suggested if I was you I would get approved to be a carer as this takes a long time. Even if you want to care for your own family it still takes time (many months) and the kids would be placed with a temporary carer while you get your approval.
Good luck, I hope these kids are safe and find a safe loving home with you.