I would suggest using a little bit of psychology this adventure will be a negotiation, the two of you will probably not formally sit down and lay out the rules of any reconciliation but each encounter, be it by letter or in person will be a very complex dance.
I would suggest finding common ground and perhaps even an ally in your Son In Law. If he is an unbeliever then he knows how ridiculous the whole thing is. I would reach out to him privately. Ask if you can meet him somewhere and buy him beer or lunch, something light. If the two of you have bad blood, wash clean of it right then and there. Let him know that you have always loved your daughter and her family and tell him you are not trying to stop her from doing anything BUT you want to know your grandchildren and your daughter. Ask him how he thinks she would react to you making contact. It could be that she is ready for this to happen herself but does not know how or is not willing to make the first move. It could be that he knows a nice card through the mail would be more effective than a in person visit. On the other hand she may need an in person visit and a card could shut her down further. If he says its not a good time, respect that but give him your phone number and email address so he can reach out when a good time comes along. It could be that he could support or urge your daughter to accept contact with you. Keep in mind that Sisters are instructed to subject to their husbands. We wouldn’t want him to force her to do anything but his opinion will weigh heavy in the situation.
I would also push to either meet your grandkids at a different time than the initial visit and I would avoid arriving with a bunch of gifts for the kids without asking permission first. While im sure you will enjoy spoiling them first, you don’t want mom to think you are trying to win her kids over with gifts.
Whatever you do... leave the WTBTS and talking about it OUT of the conversation. Prove to her you are not an apostate waiting to drag her out of the cult and to her death... remember thats what she currently thinks.