1. depending on the age of the child, I may explain a bit of what sexual orientation is. If very young, I would just say that people fall in love in different ways. His friend's mommy fell in love with a man, and they created him. I fell in love with a woman, and we chose to adopt him. (I like expressing the idea of adoption as something like "out of every kid we saw, we chose you.)
2. There *is* an element of choice to it, but I cannot believe that the core of a person's sexuality is chosen. A person who has the ability to be attracted to both sexes may feel as if that is perfectly natural, and he is choosing not to reject any option. He may simply not understand how uncomfortable straight and gay people are at the idea of being with (in any sense) the "wrong" sex, having never experienced the feeling that broadly.
That said, what makes a person "bisexual?" If it only entails some degree of attraction to both sexes, then most people are bisexual, and nearly all women (I believe...if you don't agree, feel free to disregard that statement. Female erotic plasticity is not the subject of this thread). But I do think that what a person *values* in their attractions is, if not an actual choice, almost certainly a result of experience and very subject to personal interpretation.
So, assume that my best friend and I have *exactly* the same core sexuality. When we look at or interact with the same person, we feel exactly the same thing. Well, I identify as gay, but I still do feel *some* attraction to certain men. I just do not attach much value to the attraction that I feel. I can't imagine ever living with a man, or even maintaining an ongoing relationship. The physical act of ....well *certain* sex acts with men do seem appealing, and some men are nice to look at. As I can't imagine *actually* having sex with somebody I wasn't in a relationship with, that superficial attraction means very little to me, so I put that all together into "not really interested in men, beyond the physical. Interested in all aspects of relationships with women. That means I'm gay."
It's entirely possible that my best friend, feeling the same thing, but being more open to the idea of causal sex, would see it as, "Like to go to bed with men sometimes, but prefer women. That means I'm bi, but lean toward women."