What if your son or daughter gave you a birthday cake that says I hate u you ******* shitty parent worse I hate u?

Id talk to them. There isn't a more clear message your not listening to their concerns.

talk to them and see what the concern is

One year, my dad's brother and two of his sisters took my grandma a birthday cake that said "Happy Birthday, wh*re" on it. They invited my dad to join them, but he had to work.

My grandma was an opportunity seeking con artist. They got this for her, after she broke up someone's marriage (a family friend, no less). My dad and his siblings were all adults and out of the house at the time this happened. I think all of them were married too. This wasn't the first time she did that, she was also a chronic cheater. My dad was oldest of 10 kids and they all have the same father listed on the birth certificate but my dad's dad actually fathered maybe 5 of them. The other 6 kids at the time were all too young.

What flavor?

A child would have had to be raised in a dysfunctional home with parents that were very abusive to the child for a child to give a birthday cake like that. I would speak to a therapist asap with the child. The child has a voice and the child needs to be heard. And the parent needs to see the damage that was done to the child.