This is a very difficult area and one an ex will manipulate. I had all the same issues and it's very hard to stop it. If you have something like communication is only to take place if the children choose to message her, all that will happen is she will instruct the children they have to. And if she is an alienator she will make them feel rubbish if they don't do what they're told (or even punish them).
All this goes over Cafcass head because they just don't get what an alienator is. I would suggest he just asks for contact with the Mother must be limited to avoid pressure on the children and disruption of their time with him. And maybe have that in a clause in the order.
eg phone contact will be limited to once a week and the Mother must provide the Father with the passcode of the childrens devices.
That way she can't pressurise the children to do it more often. you can't do much about a tracking device as most phones have this built in (with the excuse from her it's in case the phone is lost eg) - like find my iphone. Apps like snapchat etc can show location as well.
You can't fight all of it so be selective. The main thing is to have a good order for as much time as possible, and ideally shared care so it gives him a bit more power/equalises the power. And then you can manage things your way within that time. The other thing they do is instruct the kids on your time (either before they come or by phone) so they feel anxious or conflicted. You can help with this by saying - in this house we do it this way. then they can blame you so they don't get into serious trouble. Like saying - Dad wouldn't allow that as he said it's his rules in his house. That gets them off the hook.
So another clause that would be good to add is "any adult communications or arrangements regarding the children must be between parents and not using children as a messenger or conduit".
Having something like that in an order shows, in future, there have been those kind of issues (if you end up having to go back again).