I have been there. I was one of 6, and my children had loving parents (as did most of my siblings) but one did not, and my parents gave most of their time ( and a lot of their money) to her and her children.
Now my head said that was fair, after all my children had love, attention and a comfortable life. But my heart was different, because 'it wasn't fair!"
This is so difficult for you,t ake a breath and realise that your children have a better life with you, although their relationship with their grandparents is not as good as you or your children would like. One thing I did was to invite grandparents to my house, so they could spend some quality time with them.
Now my children are adults, and they probably have a 'good' relationship with my mum, my dad died 19 years ago. The 'favoured' grandsons don't see her much, if at all.
However, and here's the rub, it is rumoured that she has left her house to these grandsons. I don't know if this is true or not. We shall see. Although the house may well have to pay for my mum's care anyway