Miles was uncomfortable because of his prejudice (an understandable one), not because kid Keiko was in, any way, naughty. For the vast majority of viewers, this was our first confrontation with the idea of ageism. Especially confusing because it was presented in the reverse. Nonetheless, it maintained all of the controversial qualities of that prejudice and explored it from a common perspective.

And, although uncomfortable, Miles understood the nuance to complexity of the situation, and comforted his wife, despite his discomfort with her child form.

I thought it was actually a pretty good piece of acting from the both of them.

As a sidenote: Miles has, several times in Star Trek, in the metaphorical platform for working through prejudices in, depending on the situation, often an elegant manner. He’s often presented as the every man in a complicated situation, and we often get to see him work through such complicated social issues while both acknowledging painful past while at the same time evolving to the better man for understanding and acceptance.

Miles O’Brien, the most tortured character in Star Trek, suffers for the benefit of today’s society. For the benefit of all of us. What nobler cause could there be?