[AlphaBravo]

Lately these mushrooms have been growing in my back yard where there is a lot of rotting wood and shade (mainly by Sumac trees). I have no idea what they are or if they are in any way dangerous. I'm concerned as I have 3 young dogs that I fear will consume these although they seem to have no interest at all. I go out twice a day and remove them anyway but some info on them would be great!

Thanks!
Joe

[jhorn]

Looks like Agaricus campestris to me. AKA meadow mushrooms.
They are a close relative of standard white button shrooms from the store.
You can confirm by breaking off a cap and putting it gills down on a piece of white paper for a few hours. It should leave a dark brown/black spore print. Try it. They look very cool.
Agaricus campestris is a good edible, but please don't unless you get more than my 2cents on the ID.
I wouldn't worry about the dogs though.

[Jochs]

It should not bruise yellow if it is Agaricus campestris.
If it does, it may be Agaricus placomyces, or Eastern Flat-Topped Agaricus, which may be slightly toxic and is reported to have an unpleasant taste.
Both have a chocolate-brown spore print.
Here is another helpful link:
http://www.mushroomexpert.com/agaricus_placomyces.html