Just to be clear by the age of 3-4 a child should be aware of the concept of not taking what belongs to others including their pictures.
If your SO was fine with their phone being used to take nudes of them it would not be an issue. However in your 1st comment you state they are not.
The kids is old enough to understand boundaries and the word “no”. If that behavior is limited to your own household then fine you do you. It never is though.
It should be very obvious that I have kids just as well as it is obvious that you seem to be outsourcing parenting.
Of course kids are different, that’s true for every living being. Of course setting boundaries is hard, in my observation it requires way more that 2-3 times teaching - sometimes way way more. Especially when it’s an important thing that’s also fun like „don’t run across the (busy) street” or “don’t touch the hot thing” or whatever is going on with your phones.