I don't think you have to hide that you care for and value any partner other than a spouse. Kids adapt to how their families operate without any trouble. They won't necessarily understand exactly how it all ties together until they get older; they'll just know that there are more people mommy & daddy love, and as long as it doesn't threaten their security, it's OK.
My children are no longer small. My youngest is 12 and knows I have a wife and that I was also seeing her mother from before her 11th birthday to last autumn, and that my wife (the step-mother) knew all about it. We didn't explain it to her and it never came up as a problem. Since her mother and I have split, I don't spend as much time at their apartment when I visit, which I suspect she misses (she likes having all the parents together at the same time). My son was old enough to tell what, exactly, was going on and he didn't bat an eye, as he's been exposed to all manner of loving arrangements over the years.