my ex-wife... .instructed them to keep this (contact with my family) from me.
This puts the children in the middle and unless there is good reason for the secrecy (self-protection, etc) then it puts undue
stress on the children. Do they have counselors? If the ex refuses to allow counseling you can pursue this in court, courts love counseling for the kids, but probably too you can alert the school counselors in the meantime.
If talking by phone and email is difficult for the kids, hey it would be hard for me too, would video chatting overcome that hurdle?
As for your relatives, you would know better than us at this point why some of your relatives are causing problems. Partly it may be that the kids have never met your relatives. Just a thought, if you do see a need to ask your relatives or some of them to step back for now (and the kids' counselors agree) then you could ask them to have just a few of them maintain contact. Which of them would be the most helpful? Maybe you could suggest a short list of the most helpful or meaningful ones.
What I'm saying is that stopping
all contact could be interpreted that you have concerning issues. Around here many members describe that not only have they been blacklisted but also every one of their relatives are blacklisted too. For that reason blocking all relatives, even your own relatives, could be perceived as a red flag, at least on the surface.
I'm sure others will chime in with their thought and ideas as well.