I think that rather than some article about how its "too soon" for kids to meet OW, you should focus on the legal aspect. That is, what do the laws of your state say about visitation and cohabitation? In many states having overnight visits is prohibited if there is cohabitation. I'm guessing that's already what you have written out.
I think you should share that info, not as a threat, but to illustrate what society expects.
