> 先生にこんなお孫さんがいたんですか‌

Could someone explain to me how it fits in here and why it works?

I'm still confused about why they use the ''に'' instead of ''は'' here like
this:

> 先生はこんなお孫さんがいたんですか‌
CC BY-SA 4.0
2023-02-13T11:30:07.340
98589
2023-02-13T11:36:55.760
2023-02-13T11:36:55.760
55697
55697
question
particle-に
How does に fit in here?