I'd guess almost any adult can understand, enjoy and appreciate Wolf Children, as well as most Ghibli films. However, I wouldn't really call those good introductions to anime, as they aren't really in any way connected to the common style and tropes of anime, if there is such a thing. That said, I don't think you'll ever get the average parent to enjoy something like Lucky Star, Madoka Magica or Kill la Kill, no matter how gradually you build up to them.
Basically, just try to show them things that are somehow similar to what they already like.