Perhaps for the inlaws not specifying visit lengths thing, you could try "Just so you know, I'm planning to be busy on Tuesday night next week, so I won't be able to have dinner with you/entertain you/hang out with you." or "If you still plan to be here Tuesday night, it would be great if you could babysit, but if you won't be here, I really need to line up a sitter now." Heck, if they're flexible, try to make them flex in a way that is more convenient for you rather than less.