My Dp is someone I have known a while, before we became a couple.
My dd ( was 13 when she met him and now 14) and my son (was 6, now 7) met him almost immediately as he is a relative of my friend. My friend is so close to my kids their call her auntie. As far as they were concerned he was simply auntie sues relative.
We have now been together since May, the kids still don't know we are a couple. They know he helped me move house and decorate their rooms, they know he pops in on his way home from work occasionally.
There is no need for them to know he is my boyfriend. They like him, don't mind him popping in on occasion. I see him properly when they are at their dad's.
Their dad, on the other hand started seeing someone in May (I suspect he was seeing her longer but that's not my business) and they all moved in together 2 weeks ago. Her kids as well. It's been a nightmare. The kids lives have become so different, so fast they can't cope with it. My youngest is refusing to go back. He doesn't want 2 new brothers or to live with loads of other kids. As he sees it. There's been issues since their dad introduced his new girlfriend as his new girlfriend over dinner and didn't give they time to process it.
My point is that just because you know it's serious, you know him well, his kids deal with it ok doesn't mean anything. You need to take this at your kids pace. If it's serious and it's going to last, what's the rush?
His kids are a lot older and they are different people. It has no impact on wether your kids should be ok.
I get that you are happy and the temptation to throw everyone together and have one big happy family is hard to ignore. But slow down.