I allow my greys on my shoulder, but in the beginning it was for a very limited amount of time. It is their favorite place to be, and they will get dominate if you allow it all the time. As the bond and trust develops, you will find that you will be able to allow it when you want. But as the others stated, YOU must be able to train them to step up, or down on command.
One of my greys will step up all the time when asked, (I've taught her that since she was 10 weeks old) my other one doesn't know the command as well, gets confused when I ask her to step up off of my shoulder, and will only get off after I say step down. She is learning very quickly tho.
There will always be the risk of a bite, moving to quickly, they may startle. Getting too excited, feeling that they are more dominate over you. All things to consider and be careful of. I would be extremely careful of your 7 year old and having your bird on their shoulder. They are louder, display more energy, and greys are easily frightened:evil: by those things which could lead to a bite!