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This article will help with that: https://www.gardeningknowhow.com/edible/vegetables/mushrooms/grow-mushrooms.htm

The mushrooms will not harm your plants and can even be beneficial.
You can remove them if you want or just leave them.

https://www.gardeningknowhow.com/ornamental/fungus-lichen/mushroom-identification.htm

These can in fact be toxic; however, most dogs do not try to eat them, though they can sometimes become ill by just licking a poisonous mushroom. Children, on the other hand, should be warned to stay away from them and by no means ingest them. Symptoms of mushroom poisoning can range from mild vomiting and diarrhea to severe digestive problems to complete liver failure. The best thing to do would be to eliminate them altogether before this becomes an issue. The following article should be of some help toyou: https://www.gardeningknowhow.com/lawn-care/lgen/eliminate-mushrooms-in-your-lawn.htm

The roots from the tree are likely decaying, and may be contributing to the mushrooms. But, the more likely culprit to the mushroom issue would be additional moisture in the soil. Now that the tree is no longer taking up water from the soil, the mushrooms have enough to live on. You can treat the area with a fungicide. That will help reduce the mushrooms. Reseeding the area with grass will help too. For additional help, follow the suggestions in this article: https://www.gardeningknowhow.com/lawn-care/lgen/eliminate-mushrooms-in-your-lawn.htm

Here is an article that will help you.

https://www.gardeningknowhow.com/lawn-care/lgen/eliminate-mushrooms-in-your-lawn.htm

This will tell you why they have appeared and also how to get rid of them.