How do you let you mum-in-law know that you will be calling the shots on the upbringing of your child (without sounding rude)?

As most of the people have very rightly mentioned, listen to them as there's no harm in that, you might come across a really good advice or if not at least you are making your elders feel that their experience is valued by you. But ultimately you should do what you think is right for your child, don't bend under the pressure. All the best!