Kopikatsu said:
If he doesn't understand it...does it matter if he sees it?
You'd be surprised how subconsciously perceptive children can be. They might not understand something, or even try to, but one day they'll see that one thing, and everything they've seen will fall into place and BAM! they get pregnant.
Okay, slight overkill, but just because they don't understand it, doesn't mean they aren't processing it or putting it away for future reference (trust me, I was one of those naive kids who had to figure everything out for himself (my parents always answered my questions, but the more subtle things, and the less than polite things I figured out from observing) and I can tell you that every little bit adds up).
OT: I'd say that at that age there are two main possibilities.
1. They understand it, but are too young to handle it maturely, in which case they shouldn't be watching it.
2. They don't understand it, in which case they shouldn't be watching it unless you want them to learn about it from television.
Obviously number three is that they understand it and are mature enough to handle it properly, but I'd imagine that this is an anomaly in 6 year olds, so unless your cousin is some child prodigy, I wouldn't count on it.
EDIT: I feel I should add this last bit, because I feel it is important. If a responsible adult/parent is there, that the kid trusts, that parent/adult should decide whether or nto the kid watches it, and if he does, they should make sure that the child understands the context in a mature and responsible way, so supervised exposure without damage.