The answers to these questions are to be found in Aristotle's Nicomachean Ethics, book 1, chapter 9. The page number will vary according to the text you are using, but the chapter is so short that it will probably not cover more than two pages in any text.
Aristotle says that one can become happy by acting virtuously, making it the object of one's "study and care" to perform noble acts. A child cannot be viewed as happy because children are not yet capable of performing the type of virtuous acts that bring happiness. The same is true of animals, which can only be contented.
A life cannot be judged as happy until it is over, since many changes occur in life, and no one knows until a person is dead whether their life will end happily. This is the difference between having a happy life overall and merely enjoying some good times. The point that Aristotle makes here is the same as that of Solon, whom he mentions in the next chapter: call no man happy until he is dead. Aristotle gives the example of Priam, King of Troy, who seemed to have lived a very happy life until he was an old man. In old age, however, he was forced to witness the slaughter of his sons and the destruction of Troy, before his own brutal death at the hands of Neoptolemus.