Bob, unfortunately, I tried via contacting both the son and daughter via messenger but got no response
Yes I know how disappointing that can be, but if it's any consolation (and probably isn't) I once recounted in this Forum my own success at finding the long lost daughter from my uncle's first marriage (he was himself well turned 80 at the time). He hadn't seen or heard of either daughter or son for over 30 years and despite having 5 children from a second marriage (3 girls and 2 boys -my cousins) and now all grown up and living within a stones throw of their parents, he craved to learn of the whereabouts of his first daughter. When I took on the task it was 2010 and the daughter (Linda) I determined was about 61/2.
First I learned she had been adopted by her new step parents, and given their surname. I further learned on the family grapevine (and amazing how such things work) she had married. I did not know her married surname but learned she had had a son. Then by one of life's amazing coincidences learned the son had married the daughter of a son of my first cousin once removed. This came out in a chat in Messenger after recounting I was looking for the boys mother.
Cutting to the chase -because there were many twists and turns along the way - I knew her married name and found her address via an Electoral Roll. She lived in Birmingham about 3 districts across from where my Uncle lived. (I live in Kent by the way). So time to make contact but first I had to consider my other cousins (her step siblings) and his second wife (my Aunt) did they wish me to proceed? My Aunt said she knew it was what Uncle wanted but my cousins were more circumspect because they did not want their father to be disappointed, so I was told to report back through them.
I also had to consider whether Linda would wish to be reunited after so long a time and was her mother still alive. So I sat down and composed a letter and will truthfully admit it took some half dozen re-writes, helped by my wife, a good word smith in her own right. Then I posted the letter which included a SAE. I gave my email and my land-line number and waited on tenterhooks.
Two days later about 4pm in the afternoon (I remember it well) Linda phoned. We chatted and she even recalled knowing me when we were both children. She said my letter had lifted an enormous weight off her shoulders and would love to make contact with her father again (Note I had not divulged any information about his current address, which had changed at least twice over the years). I brought her up to date with his family (she knew a little through her own grapevine as it turned out but not where he lived) and the biggest surprise of all her mother was still alive and mid 80's, although her step father had passed away.
In the end I passed things over to two female cousins (her step sisters) who made contact and went to visit Linda and her husband. Having satisfied themselves they arranged for Linda to visit their father on the following Sunday and things all went swimmingly. The best bit of it was to receive a phone call from my Uncle that evening to tell me the good news that he had found Linda at long last. I heard my Aunt (in the background) shout to him it was Bobby (the name by which the family still knew me) who had made it all possible. I heard him cry and say, yes of course, thank you Bobby and I had to hold back a tear myself.
A wonderful memory and I subsequently met Linda and her husband at a Family wedding, as she is now part of the scene. I also learned her brother (my uncle's son) lived in Scotland and had been in the army for many years. So the moral of the story is if you do have to write, remember the old saying ..."think before you ink".
PS Both Uncle & Aunt have since passed away but Linda is still in contact with her step-siblings and we swap Christmas cards.