He didn’t say he’s certain if what will happen to her. They don’t know what’s happening back home, but one of them is freaking out anyway. Tuvok is being logical in turning to the facts that are known. And he knows a lot of reassuring facts. Does this mean his kid won’t be hit by a falling roof tile? No. But everything he does know points to the positive. It’s entirely logical to refer to what is known in a situation where not everything can be known (which let’s face it is every situation).