At the same time, I like how the t-shirt makes the photo less formal. The elements are all there for a frame-able portrait: the elegant chair, the beautiful children, their pose together -- and the t-shirt either ruins it, or brings it back to less formal territory and makes it fun.
The second photo is what I believe photographers call "a great capture"
You've caught an expression on her face that tells the viewer something about her personality, which is interesting because she's so young and, in general, portrait photos tend to emphasize only one thing about kids: their cuteness. Instead, this kid is looking at the camera with a very specific facial expression. I could say "reclaiming agency against the photographer's gaze", but this is a child who isn't deliberately doing that. Instead, it's the photographer who's allowed her expression, rather than her generic cuteness, to shine through.