I think it's actually silly to suggest that *she* should have been there.
What if *she* was a terrorist actually plotting something - her kid got wind of it - and he gave a hint? Having *her* present or even knowing would deter the truth - if there was any.
Ths is the is how the situation really is - they knew NOTHING about this kid - they went in with the thouht that he might have actually heard about a plot of some type *from* someone - maybe.
Now they've ruled that out, he knows nothing.