It sounds like they are able to speak with you and feel safe in doing so. If so, this is a very important gift to them. Of more value than specific advice is a person who listens nonjudgmentally and shows understanding and support for them and for what they say.

No matter how badly the father may abuse them, you will be wise to not criticize him to them.He is still their father and part of them will take an attack on him as an attack on them.

Instead, focus on and validate their feelings. So with regard to the father hitting them with a belt, in my opinion avoid saying things like "He is abusive". Instead, say things like "That must have been terrifying". "You must have felt so alone" , "the pain sounds unbearable", etc. Focus on and validate their feelings.