is he taking them away in time agreed by court order ? or is access agreed between you. ? Does he have PR ? Why are you 'scared' of them going abroad ? It doesn't sound as though you have concerns about their welfare whilst with him, as not concerned if he took them on holiday in this country, so is it about the specific country they are going to ?
Well - how would you react if he wanted all if that from you? I can't imagine you would be too thrilled?
Only you know whether you have the kind of relationship where you can ask those things, tbh. If he has PR and is taking them away as part of normal, agreed contact, then you don't have any rights to that information and you need to trust him (though i do appreciate it can be hard)
If you have genuine, well founded reasons NOT to trust him with their safety that is a different issue of course.
You do have to consent to the trip, but if you withhold consent, your reasons would have to stand up in court. If it is a holiday in his agreed contact time, and dc are used to being with him for overnights etc, it is hard to see what grounds you would have. If you are worried about abduction, you need a solicitor.
I am taking dc overseas, I have let their dad know in writing where we are going, when we are away, contact number in case of emergency and that we have valid travel insurance. He has provided written consent to travel and for emergency medical treatment to be given, as well as a contact number for himself. We are minimal contact but this info is to safeguard dc, should anything happen to me. I expect the same from him.
It is none of your business who they are travelling with, but you can ask for an itinery. If you draft a permission to travel letter and ask him to fill in his itinery and contact number so you can sign it, that might politely make the point that he can't just take dc out of the country.