Bring them up knowing the truth.
Mine were young when I became single, their dad never came back after the day he left. Never had a penny from him. At Fathers Day, after they'd been making cards at school, my child was about 4 then, they asked if they have a dad. I said ofcourse, but he isn't here as he wasn't good for our family.
Over the years, I'd give them a little more information. I'd point him out in the street. I'd point him out when he was outside the home he shared with his next gf. I said it looks like he has more kids now as I saw the babies in the garden. But it was nonchalantly. Not a big deal, not a longing, not bitching about the dad.
The step brother approached my kids one day when they were about 7. I said it's ok to play with them, that their family isn't like ours and they're just the dads new family, they're not our family. After a few days, they went back to their own friend groups. I didn't say how their bio dad was now heavy on drugs. That his new gf thought she'd one upped me and insinuated they'd had an affair before we broke up, that she has loads of kids to different men. They just simply didn't matter to us. I eventually did tell them, when they mentioned he looks 'scruffy', that he had a bad lifestyle and age appropriately told them dribs and drabs. Those kids are older and are on drugs now too. That's why I didn't want mine to get too close to them or think of them as family brothers.
My kids were brought up by a decent man. They see him as dad as he does the dad things with them. They've always known he's not their real dad, despite people often saying how much my eldest looks like my partner! We've been together 20 years, I think it definitely helped having a good man to step up as a role model and to fill the gap though.
OTOH, my friends 12 year old just found out she has older siblings. It's broke her. She feels like my friend has lied to her and is questioning what other secrets there are. She's self harming. It's heartbreaking seeing how she feels like the reject her dad didn't want. That everyone lied to her. I did actually sit down with her and told her what I knew after I was told how upset she was. I explained that they're likely in their 20s now and are in a different chapter of their life. That she wasn't rejected, her dad just had problems after leaving the army and he didn't handle them in a good way. They live over an hour away. She'll never get to meet her dad again as he died when she was young. I told her to wait until she's 18, then if she feels strongly that she wants to reach out, then she can try then. But she's too fragile for it all now. I don't think they'd want anything to do with her, I wouldn't want them to either, especially when she's so young and impressionable. Her siblings have had police involvement since they were little as their mum and the dad were always fighting and the kids were 'rough' trouble makers growing up. Their names over Facebook etc.
It's been a shock to her system. I'd never put any child through that. Everyone should know where they come from and in a neutral way, from day dot.
My neighbour used to call her kids dad 'the sperm donor', so did the kids! I thought it was horrible, like it degraded their existence somehow. The mum passing the bitterness on to the kids. Don't be like that. The kids don't need to hear it.