Why do I suddenly have brown mushrooms in my brand new lawn?

Sounds like your contractor did a good job. Mushrooms (calling them toadstools hurts there feelings) are a sign your lawn is healthy and getting adequate moisture. They are breaking down organic material into usable nutrients, think of it as composting in place. Some known as mycorrhizal mushrooms are even better for it. They form a symbiotic relationship with the gres roots by storing and exchanging water and nutrients. The only thing that concerns me is your son. He's at the grazing stage where everything new goes in the mouth for investigation, but you already know that. ume any mushroom as harmful for his sake. You can pick'm, kick'm or mow'm to remove them and you don't need to wear gloves, even the most poisonous can't be absorbed through the skin. In fact the most deadly mushrooms can be nibbled, tasted and spit out as a means of identifying without harm, I do it all the time. RScott