Wow! Your in-laws are completely disregarding food related allergy issues, especially when dictated by your child's pediatrician? My dd has food allergies and I would be livid if our family just thought they knew best and did not care what we needed to do to keep her healthy (and in some cases just breathing). That is inexcusable! No way would my child be spending the night with grandparents I couldn't trust to keep them safe. There comes a point in life that no matter how much you love your extended family, your kids health and well being come first over someone's feelings.
Other issues, like discipline, eating cookies late at night, well, we just deal with them. We are happy to have extended family in our lives and realize that the kids adore the grandparents. They don't get to see them often, so many of the little things slide.
I don't think the OP said her child had allergies. She just is careful to introduce new foods in such a way as to make sure her child doesn't have food allergies. I'm lucky - both set of grandparents never went against anything I asked. However, TBH, I was probably more over-the-top with my first baby than my last.

Edited after reading the PP - OMG, when dd13 was about 8, my mom showed me one of my "lists" - we had a good laugh!