Even if you do prefer one grandchild over the other, you should never tell their mother that. 

None of us likes to think of ourselves as someone who would rank other people in their head. But come on, we all do it. We all have a favorite sibling with whom we like to hang out more than the others. We have best friends, friends, and acquaintances. It’s okay to feel closer to one person than another, and depending on the situation, it’s okay to spend more time with someone you’re closer to. The situations where it’s not okay to do so involve parents and children. One parent might be the one who takes their kid to their baseball games because they like it, but that doesn’t mean they should never support their other kids and their passions. It doesn’t matter if they like baseball better than the sports their other kids play. It doesn’t matter if a parent secretly likes one kid more than the other. That’s not a good excuse to ignore their other kid. Parents should be aware of their biases towards their children so that they can correct them.