Surrogacy isn’t immoral. Moreover, it would be better if you would tell the origin. But, it solely depends on how you want them to know. Most importantly, your family knows about it, and then the children. Because later go things pretty awkward. The real deal is to compile the information like a story about the origin or birth according to the age of the child. You must be not sound ashamed of it. Lastly, you may have to be friendly and open to answering the questions any time. Because the more you will be open to communication the more you will be able to voice your intention. Work on to build an honest relationship with your child from the beginning, this would build the trust. Hope this has helped. Good luck!!