To feel extremely upset about my granddaughter?

And her attitude towards me?
She is 9 years old, extremely confident and strong willed.
She likes things to go her way and gets upset and moody when they do not. For example. I told her we had to leave the shops earlier as it was getting late. She did not like this and once again, I was subjected to her moods. This is when she does not speak, sulks, slumps down to the floor once back at home and sits there with a moody look on her face. Sometimes she will get my other grandkids involved by closing the kitchen door on me and telling them all about it. Almost trying to gang up. My DD and her husband seem to think it is OK and just what kids do but I can't help but think she is too old for this and that there should be deeper exploration of why she behaves in this way.
Once again I am almost close to tears by her behaviour. Aibu to not know what to do here?