Why am I seeing my kid's search history?!

In the past several months, I have figured out that I can see my kid's search history. I have no idea how this is possible and unlike other parents on here, I don't want or care to see this. My kid is 21 so I don't need to see what they search and even when they were younger, I still would not have wanted this. The concern is also that they might be able to see mine. Luckily, neither of us has anything to be concerned about the other seeing, but it's still a concern. I don't believe my kid has figured this out yet, but it's only a matter of time. Does anyone know how this is happening and how I can turn this feature off?

We have a family account through AT&T and everyone has iPhones (phones involved are 13pro & 11), but everyone has their own Apple ID. I'm not sure what features the phones (or AT&T?) have that could enable this to happen.