Ugh! Unfortunately I don't know your state's laws or the decree's wording, etc.
I believe at least a call to your L is appropriate, just to check "what happens if X...?"
bearing in mind that you don't want your kids to be homeless. Don't let that paralyzed you from knowing the answer.
As for the kids and something being "amiss", well, even if the OM is an alright guy, of course your kids will be uncomfortable with him for awhile.
They may have retained hope for a recon, (which is not eliminated by this, but they don't know that)
and or they may wonder how you feel, or they may not have warmed up to him yet.
I think that your former spouse being with another person is always a challenge.
Is your D final yet?
M: 57 H: 60 M: 35 yrs S30,D28,D19 H off to Alaska 2006 Recon 7/07- 8/08 *2016* X = "ALASKA 2.0" GROUND HOG DAY I File D 10/16 OW DIV 2/26/2018 X marries OW 5/2016