Yes. For not yet understood reasons, children do not seem to develop symptoms from covid19, but they can still be carriers. Combined with the fact that schools are essentially giant petri-dishes for disease, and old people are particuarly vulnerable to covid19, means that sending a child to grandma is essentially playing Russian roulette with her life.

I would go so far as to suggest that if she can take care of herself, grandma should self quarantine as much as possible for the duration of the crisis. People do not become less dangerous just because they are relatives.

After the family spent 2 weeks in self quarantine, visiting grandma is probably fine.