I am in almost exactly the same situation.
My boyfriend was a friend of mine and my exh's and our children have known each other for their whole lives.
However, although the children are aware that we are together (and have been for 5 years), it makes almost no difference to their lives. We have our own houses, we talk a couple of times a day, but we really only spend time together when the children aren't there.
Once a week, I will stay at his when his DS (14) is there (which only started about a year in) and we get on great. He is one of my favourite people, but I won't impose my children on him and I don't try to be part of the 'family'. We made a decision not to blend our families quite early on.
I take my DC on holiday and boyfriend goes on holiday with his.
Once a year, DP and I go away by ourselves. We don't have big family holidays with all of the DC.
We do go to each other's big family events, but we keep the children separate. So DP came to my Mum's birthday party and I go to his family occasions, but it is very much as 'Mum's boyfriend' or 'Dad's girlfriend' and the children are secure that their lives don't change at all as a result of our relationship.
One day, we plan to move in together, but not until the children are grown up.
I know it's unusual and people don't understand how you can have a serious relationship (and it really is!) without living together or blending families, but it works for us and keeps a lot of the complication out of it.
I'm not suggesting you do the same, OP, but I think your DC need some reassurance (through your own actions) that your relationship may be a permanent fixture, but that it doesn't threaten them or change their lives at all.
Just a thought.