Forgive me---you did say that the kids' therapist or pedi would call CPS, correct? That's good news--I'm glad to hear it.
I do think that until it's determined that there will be no investigation, the info on the Nooks needs to be preserved. Actually, I think it should be preserved in any event--because at some point, enough evidence will be amassed that SOMEONE, somewhere, will pay attention.
You can preserve the data so that, even if your ex chooses to scrub things later, you have evidence.
I know you've been "talked" and "talked" to death about this, and it's NOT my intent to add to your burden. Please---feel free to bypass this long post. I do share some info from my daughter, who works at B&N--it might help, but is not something you have to tackle today or tomorrow or even at all. I just want to help. Again, I fully trust your judgment. (The main takeaway from Barnes & Noble, if you want to read just one sentence rather than the whole missive: You can save the info from the Nooks to the cloud to preserve as evidence.)
Needless to say, I'd suggest holding off on making changes to the Nook until you preserve any potential evidence that might EVER be useful.
I would not ask your ex to modify his Nook settings or separate accounts just yet. Nor would I change the settings until I'd archived things the way they are.
Don't tip your hand---let him hoist himself with his own petard. (Well, I'd rather he be hoisted by something else....) He can delete everything in a keystroke, and prolong your battle to protect your babies. Best to be one step ahead of him and get the data archived.
I know there are practical matters associated with withholding the Nooks from your kids long enough to figure out whether they will be used as evidence, and to archive evidence from them. I know the devices are not your property. But he's placed this evidence in YOUR possession. It's a sick, sick gift--but one that might ultimately help you protect your kids. And it only will take a quick trip to B&N (if that) to preserve the evidence.
I know that your kids will want to play the games Dad gave them--any kid would. If you have a Smartphone you can download the Nook app and let them use your device to play the games. Or you can get the Nook app on your computer. That way, they won't complain to Dad that they can't use the Nooks--which will buy you a little time. And a little time is all you need. Maybe little enough that they won't even notice.
Once you archive the data, you can ask your ex to separate accounts so your kids will be spared. But get your evidentiary ducks in a row, first.
My daughter sells Nooks at B&N. She can make a device safe for your child---but only until your ex makes it unsafe again with new porn (or until they download what's been archived to the Cloud to their devices again). For real safety, they need an account separate from his. Given his grooming history, I'd argue that the account needs to be one to which he has no access. I realize this is grossly unfair to you--it's an expense you did not choose to incur.
I'd make a trip to B&N and have them archive the data to your Cloud. Once your ex knows something's up, he can delete the information permanently; you want to save this, in its current state, ASAP. You probably can do this on your own, but I just spent 10 minutes with my Nook, and even though I'm pretty tech savvy I couldn't figure it all out, so I would want help from a Nook manager at B&N.
Once you've got the data archived, then you can breathe a little more deeply. The evidence will be preserved. You can then let things unfold with CPS or the police or whatever, and ask your ex to separate accounts. I'd check with the lawyer about this, but I'd wager that because they were gifts to your children, YOU can close the account and create a new one, if he won't.
Again, I know you are doing your best, and will continue to do so. I really do trust that. I am only sharing this in case there is a tiny nugget of something that might help. I know you've examined all angles, and that this is unlikely--but ...well. Your kids. You. I care, you know? That's all---I care. I don't doubt you, even a little.
Okay, a too-long addendum, and I apologize in advance. I texted my daughter the following question, and her response follows--it includes information I did not think of, involving the aforementioned saving evidence to the cloud. It might help a little. Again, your call--take what you need, leave the rest. We all do trust you. This is not about that.
Me: Hypothetical question: you're working Nook and a mom comes in with her two kids' Nooks, which are linked to her ex's account. You find the ex's porn on one child's Nook. I know that you are a mandatory reporter. Would other B&N employees be required to report to CPS? Would they scrub the device--and if they did could it be forensically recovered if needed?
DD: I know I would have to report. I don't know about other employees, but I will ask a manager. (My addendum: the manager said that employees are not mandatory reporters; if they choose to report, they are supported by management.)
We can reset the device (anyone can, it's in the settings) but if you just delete it, it's gone forever. You can archive it to the Cloud, but then it can be re-downloaded by the kids. Best bet, in my opinion, is to deactivate the device so it's no longer linked to the ex, start a new account for the kids (they'd have to restart their libraries, though)....
DD later: Manager just said the same thing I just did. Best bet is to re-register under a new account and rebuild the library so there's still evidence (my note: he can delete this at any time if you don't save it now) but the kids can't see it. And report to CPS or lawyer.
As always, a million hugs to you and your kids.