On the surface, children may appear oblivious and clueless about what is happening around them, but that does not make them ignorant or unaware. Unable to articulate and express their feelings well, you as a parent must play the role of helping them understand and speak up, especially when there is a concerning issue.
In this case, the Ukraine-Russia war. Your child may not have all the details, but with the little information they gather from adults during table talks, news and the tension in the air, their radar is able to detect that something is off. And the truth is, they are just as bothered as you are.