If you want to know about a student's artwork, just ask them! Asking about their art in general terms will often illicit real and thoughtful responses.
I learned early in my career that a simple, "tell me about your picture" is a very powerful conversation starter. If it is obviously a portrait, or landscape, the artist will start describing colors they used, share a story about the image, or even just blatantly describe their process. If you have no idea what is going on in the child's art, a "tell me about your picture" will give you the answer.
It can be damaging to ask specifics, or even comment on what you think is going on in a young child's art. "I like that picture of the cow!" could bring a kindergartner tears. That wasn't a cow, it was a killer whale, and you just pointed out what a terrible terrible artist that 5 year old is. (This happened to me while I was student teaching. A powerful lesson for me.)
Informal interviews can also be used as a formative assessment. It sounds like small talk, but the briefest of answers can clue you in to what the students have learned.
During studio time, I walk around with a clipboard and a stack of the Informal Interview forms. I jot down the child's name and occasionally jot down specifics of their answers as they talk. They do not mind when I start writing - actually - they kind of like it. I keep the stack of interview forms so I have a record of who I'm keeping tabs on, and who might not be getting enough personal attention.