Another consequence of keeping your kids on your plan is that you have access to their healthcare details within the account, which one or both parties might not ideally want.
We also have a wraparound policy thru DH’s work that covers all out-of-pocket expenses from the primary insurance, so I have to remind the kids to send me receipts for co-pays and other out-of-pocket expenses because submitting that info is not straightforward, and even if I taught them they wouldn’t do it (plus then they would have access to parent healthcare details.) So, in an even more immediate way I know if they went to the ER/urgent care, visited X specialist, was treated for X, saw a therapist who doesn’t take insurance, etc.