Yeah but we have no proof, and the mum will lie to protect her dad, if the police show up and then leave and do nothing because they are all pretending then they will be suspicious 

Heya.

That is unacceptable and reprehensible of them.They could maybe tell it to a person (an adult) they trust, for that, there is no proof required in order to tell it to someone they trust.  If they're hurting your mom, they could also just see the injuries, which is also a form of proof.  They can help with reporting it to the police. 

It's already good of them that they've told it to you. I would certainly tell her that it's great of them that they've had the braveness to inform you of this situation. And tell them that you'll always support them, no matter what. To make them feel being seen, you know. That's really important for people who are dealing with difficult situations.

As @Eenminimalgyall said, child protection is really important in the Netherlands. There are a lot of organisations in our country who are fighting for child protection, and they might wanna aswell contact them. Their phone numbers are usually listed on their websites. And they can always chat or call with this website. So maybe they can make use of this too.

I think passing this on to the police is the best