Okay, thank you for the information. The main concern is their physical safety, and as you have no worries about that, then yes I would let them say goodbye to him. Especially, when the 9 year old has voiced that she would like that. If one of them does not want to go, I would not force them to do so. It they are neutral, then they could go.
Tell their Dad's mother that the goodbye needs to be light and positive. No heavy stuff can be put onto these children. Daddy can say he is sorry that he took their stuff, that there is no excuse for it, and Daddy is sorry. That it's Daddy's problem that he has to work through, and that they have done nothing wrong at all! That is all that needs to be said about that.
And then, allow the kids to ask Dad any questions they may have, etc. Dad needs to be strong and positive and make these kids feel reassured that he will be okay, and that they will be okay too. Be sure he doesn't say anything to them, like "you need to take care of your mom now" or anything that puts undue pressure on them. They are children who deserve to be just kids, without major worries.
So, yes if they are safe AND Dad can handle himself rationally then it should be okay. I would keep the visit relatively brief. It's important that their time with him ends on a good and strong, reassuring note for them... So, do talk to Dad or his mother about guidelines for this visit. If you feel that it's going to be awful, with no guidelines in place, and anything could be said, etc. than I would simply have them say goodbye to him over the phone, where you would have much greater control over the situation. Please click ACCEPT button, so I'm credited for this answer. I wish you the best. Thank you,