My info is quite old and does not relate to World Vision, but you can draw your own conclusions wrt 3rd party collectors. I had an acquaintance who did street collections for 3rd parties on behalf of collectors. This is what he said happened:
For every Euro collected, the 3rd party took 50c as profit , the collector got 20c, 10c went to the 3rd party for admin expenses , leaving just 20c for the charity. This method of collection gave poor return for the charity, but it was 20c that would otherwise not be collected by the charity. Maybe the 3rd party you're dealing with isn't as heavy on the charges, but you can be sure they're not doing this for free.
I now refuse to give anything on the streets, unless I am 100% confident that ALL of the donation goes to the charity.
If you want to sponsor a child through [broken link removed], my advice is to do it directly.