Im not any kind of expert but I would have thought that at 12 and 15 (in particular) 1 it would be impossible to physically force them into contact if they really really didn't want to without some kind of restraint or assault and 2 the judge would listen to their views if they said they didn't want to go.
They need to speak to CAFCASS. The children's guardian is the only person who can represent them in court.
Encourage them to speak openly about what they have seen/heard/felt when they have been with their dad. Help them find a way to communicate with the CAFCASS guardian- a list, letter, email.
The children need to evidence the harm that their dad is exposing them to - ie I saw dad hit my step sibling, this made me feel...scared/sad/angry/worried.
I would strongly encourage you to find a counsellor for your children so that they can talk through their feelings without feeling under any pressure. Their school may be able to provide one, or your local youth services will. It will be free.
I would suggest that their dad is not going to change. If they want to see him then keep the contact safe for them - make sure it is in a public place, with a structured activity (lunch, bowling, cinema) for a short period of time ie 2 hours. He is unlikely to harm or bully them in public. They are at most risk when they are behind closed doors.
Teach them that they can leave the contact at any time, that you will collect them immediately if they text you and ask you to.