How to I ask my in-laws to stop spoiling my child?

Nothing goes wrong with communicating well with them. Tell them how you feel and make sure to set guidelines but do so in a way that they won't feel offended or disrespected I totally agree with your battles about spoiling, but then again there's nothing wrong with your in-laws giving them things from time to time, but there is a certain extent for them to give. Just consider letting them know of your concerns, I'm sure it'll turn out well!