Oh, boy, can of worms here, and an often discussed topic. All I can say is that being left handed is a natural thing and one should not go against it because of trivial obstacles such as lack of instrument choices and or the inability to play an RH instrument at a party. . . There were two RH guitars in my house growing up, and since the first day I grabbed one at age 10 I naturally flipped it - I just couldn't imagine playing it RH. So I learned to play on a RH guitar left handed until I got my own guitar and flipped the strings.
It's been a pain but fun being left handed and trying to find the right instrument, and nowadays when you can find a LH Fender pretty much anywhere in the world, I don't see what the problem is. I know it gets hairy with orchestral instruments and what not, but making a LH kid play RH without even trying to find a LH first, it's a disservice to the kid. I know that most schools have a poor budget for instruments and getting a specific instrument LH could be impossible. In that case, I think it would be okay for the kid to just start playing RH, but if you suspect that the kid has a lot of talent and is being held back by the RH instrument, or if the kid shows a real interest and talent in music as time goes on, I would try to find a LH instrument. There are some amazing LH people who play RH, so it's definitely doable, but who knows how would they play would they have played LH. . .
There's still so much misconceptions about hand orientation. Funny story, my Mom tells me that when I was a toddler learning to grab a spoon, since day one I naturally used my left hand, and an old aunt (and old fashioned) who babysat me used to make me grab the spoon with my right hand and I would throw it and start crying and simply refused to use my right hand. . . apparently my aunt thought that using the left hand was wrong and could lead to problems. . . that was over 40 years ago, but I'm afraid there still is a lot of people in the world who think that being left handed is wrong. (Not related, but I do think that driving on the left side of the road like in the U.K. is wrong!
)
In sum, don't fight it. A person's natural hand orientation should be respected and encouraged.