I agree that it’s hard and perhaps uncomfortable but very often it’s in your own perception of things. So you for example might be embarrassed if your house isn’t in perfect order or your kids have chocolate all over their faces, while your in-laws might feel perfectly comfortable with the “mess” and love seeing your kids in this funny state. And if it doesn’t happen too often your kids will survive a slightly later bed time and benefit from spending time with their grandparents.
So examine your true reasons of discomfort, it may be you actually need your privacy at that hour, or perhaps you feel you must be the perfect in-law and bake a cake and do a major clean-up before having them over. If it’s the latter, the more you accept them unexpectedly the easier it’ll get down the road. Hatzlacha!