Hi there, I hope you are well.
In regards to your questions, it all depends on whether you look at it from a reality or a fictional point of view.
Kaa the snake is described as an Indian Python in the books. I'm not sure which movie you watched, however most movies also follow suit and design him as being based on an Indian Python.
These snakes in real life do act very much like Kaa, they wrap their prey up until they suffocate, they then proceed to swallow their prey head first, swallowing them whole and slowly digesting them over time. However, these snakes prey on small rodents and mammals, and sometimes lizards, birds and reptiles. The snakes are very long (sometimes up to 25feet long!) however they don't have much girth, therefore an Indian Python that swallowed a human whole would probably split from the inside out due to the huge size of the human. If it didn't split then it may be stuck in place due to the sheer weight of the Human, this would leave it at risk of other predators that may be after either the snake or the remains of the corpse. Also Indian Cobra's unlike Kaa, are not very aggressive, they will usually scavenge foremost, then hunt if the right opportunity arises. All of these factors point to if being very unrealistic that the snake could or would eat a human.
However, from a fantasy point of view, if we look at Kaa, it is completely acceptable to believe that a snake like Kaa could and would eat Mowgli. It is far more aggressive and is seen to target humans on a regular basis through the movies. The snake is also of a much bigger size compared to Mowgli's, Snakes are moan for their very stretchy flexible skin, therefore a snake of Kaa's size could easily swallow a boy of Mowgli's size without fear of splitting. From this point of view the answer is 100% yes! The kill would have gone something like this. Kaa would have wrapped Mowgli up as we see him do, he would then have proceeded to squeeze Mowgli until he died. The snake would then have swallowed him whole, head first, before slowly digesting Mowgli. The snake would likely have had a huge Mowgli shaped bulge somewhere down his tail as he slowly digested him.
I hope all this has helped
Thanks for your question