Had the same process when we went through this - it’s a safeguarding and MH check to ensure that your other child’s needs are assessed and catered for. Your other child will have been impacted by the attempt, they may be in deep emotional distress and need counselling and support. Am a little bemused that you can’t see this?
Then there is the fact that they need to double check that the reasons underlying the suicide attempt are not to do with parental neglect or abuse, ie that your other child is not also at risk. They can’t have that conversation with you present. They usually also see them in their bedrooms, so that they can assess them in a safe space and also see the level of care/cleanliness etc
Again, this is a pretty obvious step to take, a necessarily and common sense check, with a family where one child has attempted suicide and the other, statistically, may be at risk.
I was grateful that they cared enough to check on the wellbeing of my younger child and recognised the impact that everything older child was doing. Siblings of distressed mentally ill children are often overlooks and neglected by the system.
Embrace it as an opportunity to ask for more help - for both your kids. We had a wonderfully affirming report at the end of ours, a one to one youth worker in addition to being allocated a keyworker, and our younger child was ear marked by the GP for any additional support they might need whenever we called re their anxiety/migraines etc. It doesn’t have to be a negative thing.