I'm so sorry for you Jane. I know how you feel or can sympathise with you. My DD has told me not to attend her wedding next year as I was upset that she wanted me to have no part in it. Again, her fella has no parents now, her dad doesn't see her so I am the only parent/grandparent to their two sons. I have tried my best, with disabilities to make up and compensate for the fact that I am the only parent. But to no avail sadly. She didn't want me to walk her down the aisle, no speeches, no top table. I was expected to be just a guest, probably because they are paying for it all themselves. But they have had children first and I bought most of the nursery items plus clothes for her and the children. They can't have children first and then expect money for a wedding too. If they had got married first then I would have contributed, naturally and then offered to pay for just one item maybe of nursery ware. But she had me buying allsorts when she was pregnant and then, even though she had a pram for the first which was still good, she wanted me to buy another when baby no. 2 was on the way. I didn't. I have brought her up practically on my own from being a baby herself so it does hurt but I refuse now to keep hurting about this and am looking to get on with my own life. I can't change her. She wants to cut me off so be it. It is what it is.
I do feel for you though, it must have been a huge shock to discover that they had had children without your knowledge and then to relay that news to your husband and family. They don't know what they are doing. And the children, when they get older will mix with other children who have grandparents and be asking their mum and dad where theirs are? It is then up to them to either lie or tell them that they cut you off. If they want to lie to their own children, like my DD will do to hers over me (my oldest GS will remember me and my doggie probably), then this is their call. I know the truth, as you do.
It's a bloody sad state of affairs when our children who we have loved and nurtured no longer want us in their lives. I've just turned 50 and got nothing from them, not even a text. And she goes in for surgery next week. I really didn't want to give in and wish her well, but my heart wouldn't say no so I sent an e mail earlier. At least she can't say I haven't bothered.
Take some time out and come to terms with what has happened. Then think about the next step, but I really don't know what you can do if you have no number and did you say you don't know where they live now? Have you any idea where either work? Or through social media sites? My daughter said they were moving and I've asked for a forwarding address but nothing so far. Just look after yourselves, a big hug coming your way.