They are always advantageous for kids. But for lightweight ones they are also dangerous. I don't remember the exact numbers but they are somthing like this:
1) Airbags are assumed to have saved 1000 kid's lives when otherwise they would probably have died.
2) Airbags are assumed to have killed 60 kid's lives when otherwise they would probably have lived.
So airbags save 16.6 kid's lives for every 1 life they have taken. So with that in mind, airbags should be used with kids. It is just the overreaction of people that make it seem like they are bad. They'd rather have those 1000 kids die so those 60 kids would have stayed alive. Seems dumb, but lawers can and will sue - so they put up warnings encouraging more deaths but fewer lawsuits.
That said, the back seat is still safer.