I lived the same situation and this is what I have learned. There is NEVER a good reason to favor (overtly) one child/grandchild over another.
There is NOTHING you can do to change the situation.
You cannot protect your child from hurt feelings (and trust me, as a parent, this is very, very painful). Allow your child to express his/her feelings to you, NOT the older generation. Their reaction will only be to strike back to your child, which will permanently damage your child’s self-esteem.
It is time you write a letter to your parents. It should be non-confrontational. It should only, factually list examples of neglect. ie: specific invites/turn-downs. little league games/turndowns, school events/turndowns, etc.
End the letter stating that your children have been hurt by such benign neglect and you would like them to be aware of what has happened.
You can end your letter with your love, for they are still your parents.
Make sure your son/children read the letter. They need to know that you see/understand their pain, acknowledge the disparity, and are stating “their” case to your parents.
It made no difference in my case, in the grandparents actions, BUT, it had an enormous impact on my children, knowing I knew, understood, and truly empathized with such a lop-sided relationship.
Before Rosh Ha’Shana is a wonderful time to share your family’s feelings with the rest of your family.
Good luck !!