I really do think the “your child, your rules” maxim, held out as justification for taking action is response to a complete non-issue, is unhealthy.
As long as your child is safe and happy, then why should a parent get to exert complete control over what happens when a grandchild is spending time with grandparents. As a poster said above, they do not own their child.
You don’t get that control at nursery or at school. And it’s really disrespectful, and damaging to a close family relationship, to micromanage, disapprove and reduce contact over absolutely nothing. It has grandparents treading on eggshells, always at risk of being considered in the wrong.
Imagine if that was OP’s DH, taking such a critical and inflexible approach to OP’s minor parenting decisions. Why is it okay to exert this level of absolute irrational control in this relationship when it would be called out as toxic behaviour in other contexts?
How far does it go? If my rules are that I want my child to eat 4 blue smarties at 4.07pm every day, otherwise I am going to cut contact, is that justified?
There has to be a cut off point after which this approach must be recognised simply as control for control’s sake. Control should only be exerted where there are good substantiated reasons for doing so - ie health and safety, behaviour management.
There is no way that a drop of milky tea and a bit of cake is putting OP’s child at risk of anything harmful. So this is just about her trying to show her PILs who is boss. Ultimately she is prioritising her need for control over the broader family relationship. She is the one who risks causing harm to her child by undermining this precious and obviously loving relationship between her child and their grandparents.