Family lawyer here. And mother of 4 teens who all complain in one way or another about contact arrangements. However, their DF is luckily very decent and although he will moans if they want to change arrangements he does understand.
First of all the legal position.
Read this:
www.pinktape.co.uk/rants/how-long-does-a-child-arrangements-order-last/
It may well be that your old order no longer applies to the 17 yo, so nothing DF can do by law to force her to go.
Re the 13 yo the old order technically still applies but can be varied.
You either impose a change and let him try to enforce the old order (then argue its DD's wishes) or you apply to vary it. There are typical times in a DC's life that a court would say it is natural for arrangements to be reviewed/evolve and 12/13 is one of them. This isn't enshrined in law, it's common sense.
Moving on to the practical side of things. The DDs could simply raise it with DF and say what they want, supported by you. However, DF is a conflict-based person. The DDs don't want to create conflict. They know this is what will happen.
The DDs have to understand that at their age theirs is the voice that will be heard. If they don't speak up then they are stuck with going along with the arrangements. You can't do this for them.
Think laterally. Could they maybe go to the school counsellor and ask for assistance in communicating their wishes to their DF? Or their form teacher?
What about some form of group mediation or therapy/counselling? Or just you and DF going to mediation?
Before either of you are allowed to issue an application you have to attend mediation. The person applying must attend, the other person can refuse.