I don't think lox is considered meat, but my understanding is if they're kosher enough to care about not mixing milk with meat, they aren't going to eat anything in your house because it isn't kosher for passover. To make a food that's truly Kosher for Passover, it needs to be made in an environment that's cleaned in a certain way of any trace of chametz, using dishes that have never cooked chametz.
My understanding of chametz, is that you can't have any products made with flour if the flour has had any chance to rise whatsoever. This includes any flour that has been mixed with wet ingredients, unless it's made into matzoh under strict conditions where it's rushed into the oven under the supervision of a rabbi. So, while baking powder may not be chametz, pancakes definitely are.
I think in your case (not observant Jews, knowing that they're coming to a home that doesn't keep Kosher or observe Passover) you're fine with pancakes as long as you have enough other foods to allow people to skip them. I'd have something else carby to balance out the eggs and lox, maybe a fruit salad? Kids will love that, it's gluten free, and easy to make. Maybe have a box of matzoh on hand too.