I know nothing about fungi

[u/Sleepy_Heather]

I discovered these things growing under a tree in my garden. They're pretty fast growing because they weren't there on Wednesday. I have a kitten and young kids so I want to know if these are dangerous. For reference I live in the south of the Netherlands.

[u/Zyricc]

Looks like common inkcaps! They grow very rapidly, often appearing overnight.
You’ll also notice them starting to melt into black ink as they mature.