My parents got a lot of mileage out of "we'll save this to play with later" and "let's take out the batteries now so we don't waste them, we can put them back in later when you really need them*"

And the advantage of doing it that way is that it doesn't give the MIL the opportunity to set up a power struggle where you argue with her about what your own children are allowed to do in your own house.

When Dr. Spock tells parents they need to have an air of cheerful assurance, that's not just useful in dealing with preschoolers -- it's also useful in dealing with other adults. You don't even need to say "I get final veto over what comes into my house." You just do it.