What do the sheep, rose, snake, and adults symbolize in "The Little Prince"?

It can be hard to attach specific meanings to the things you mention. Let's consider each one:

The sheep is not even a character, really, but a desire, or, at best, a drawing. It is important that the only satisfactory drawing of the sheep is not a drawing of a sheep at all, only of a crate containing the sheep. So we can understand the sheep as a symbol of unconscious desire, or of the imagination -- it something we want but can't really describe in a direct way. The sheep can also be understood as a religious symbol: for Christians, Christ is known as "the lamb of God." The picture of the crate with the sheep inside can be understood as representing faith -- like the sheep, faith is a belief in something you feel to be true, but can't see.

The rose represents love. Flowers often represent beauty, grace, and purity, but the Prince's rose also is vain and demanding. The Prince's problem in the book is his quest to understand the Rose, and to understand how to love it. 

The snake, like almost everything in the book, can have many meanings. In a Christian sense, the snake has a specific meaning as a form of the devil that tempts Eve and causes Adam and Eve to be cast out of Eden. The snake represents death. Yet in the book, the snake is the means for the Prince to return to his planet -- a means for a sort of ressurection. In this sense, it is another kind of savior.

The adults represent the everyday world in which no one has time for imagination and everyone is consumed with useless tasks. There is the geographer that can record no information because he is not an explorer, or the drunkard who drinks because he is sad and sad because he drinks. All the adults are stuck in similar ruts, and none of the things they care about have any bearing on the real issues in the book.

So you can see that it is not easy to attach specific symbolic meanings to elements in the story. I think part of the point of the book is to make it difficult to create those kinds of definitions, as a way of challenging the reader to engage his own imagination!