Fungi in house plant pot

Fungi in house plant pot

Observation - Fungi in house plant pot - UK and Ireland. Description: Would anyone be able to identify this fungi growing indoors? My little nieces are very curious to know what it is (and so I am), but more importantly: is it dangerous? and how did it get there? Thanks a lot!

Would anyone be able to identify this fungi growing indoors? My little nieces are very curious to know what it is (and so I am), but more importantly: is it dangerous? and how did it get there? Thanks a lot!