Just be honest: " A long time ago, I had a baby, and another family took good care of him. He wants to meet you and your (little brother, little sister). I think it will be fun for all of us to get to know each other."
Keep it simple. I was the older kid who was integrated into my bio-dad's family when I was 14 (my sisters were 10 and 6 at the time). My advice would be not to 'play it up' too much, other than just "here's the short story", keep it super simple. And certainly reassure your kids that you are so happy to be their mom, so glad that they will always be with you. That's a fear which might come up for some kids. I would also limit any pictures to what he looks like, now, just so they know who to expect.
May I also suggest going to meet him on your own, first, and then taking time to digest this before doing a bigger 'family' meet? He may have some questions for you, things may be uncomfortable at times. I'm not saying this to bring you down, just remembering what it felt like to meet my own bio-dad for the first time... I was really glad not to have to meet the whole family first. Find out what he'd like in this regard.