If they are poisonous and the chickens eat them, you may have trouble. I think it would be best to remove the mushrooms when you see them, for safety. When I lived in a wet area that grew mushrooms, I did the same because my cats seemed to show an interest in them and I knew I had a poisonous variety. Non-poisonous mushrooms should be fine though.
Chickens will usually avoid things they instinctively know isn't good for them. Some mushrooms they love; most of the ones in my yard they won't go near. It's been a very good mushroom Fall here; I have fairy rings and lots of 'shrooms growing on the cut stumps, but the girls won't go near them.
There's lots of other things for them to eat, though, and they free range as well as getting lots of feed. If my chickens had not much else interesting to eat, then they might take more interest in the mushrooms.
DK publishes an incredible mushroom guide, with gorgeous pictures and a really good identifying system. It's worth ordering this book on Amazon.com or getting it from a bookstore. DK publishes really beautiful and easy-to-use travel guides and lots of kids books on nature and science.