Should I tell my child my partner is not their biological parent?

Yes. I have a friend who once told her child that her father wasn’t her real dad. You don’t have to do the following, but I’d advise what she did. You could try saying, “Hi, can we have a talk? Don’t worry, you didn’t do anything wrong. It’s just about your family past.” Then sit down, and say, “You see, your dad/mom isn’t your biological dad/mom. There was somebody else.” Answer questions, explain gently, give lots of hugs, and give a lot of support. Another idea is to wait for a few years until it seems you’d both be ready for it. Or, if you want to do it now, you could say, “Try not to judge your dad/mom, but he’s/she’s not your first one. You see, can we talk about it? You had another dad/mom before the new one came along. Don’t worry.” Or you could innocently say, “That’s just like what your first dad/mom did/said/thought/saw/heard. Oh, you see, your dad/mom isn’t your biological one.” This blasts off a family talk, kisses, hugs, and love required.

Yes . The child should know the truth. Whatever may happen but it's never good or fair enough to build a good relationship on a lie.

I believe that honesty is the best policy. But you have to do what you are most comfortable with in the long run. Sometimes being calm with someone and scheduling a time to have a serious conversation helps and saying it in a nice way to keep the conversation light is a nice way as well. You have to go into the conversation understanding all of the risks but also understanding all of the benefits. You just need to be prepared and I've noticed saying one or two good things first helps bring out the bad news. Ice cream never hurts either!