Pat Kiewicz wrote:
HytestA said:

I have just moved into a new house with my wife and daughters, we
have a nice garden but its been a bit neglected over the years by
the previous owner and whilst going over it my wife noticed we have
an abundance of a strange plant that we dont recognise. It looks a
bit mushroom like but its 'cap' is hard, and when mowing the lawn my
wife said that it looked like it emited smoke or spores.

Yes, it is emitting spores. Equisetum (horsetail, scouring rush) are
plants that reproduce by spores rather than seeds. They are often
referred to as 'living fossils' as they were the dominant type of
land-plant hundreds of millions of years ago.

http://en.wikipedia.org/wiki/Equisetum

One of our main worries is for our young daughter who will be
starting to use the garden in the summer and we want to know what
this plant is, if its dangerous and how to get rid of it. Any help
identifying it will be most appreciated.

It's not particularly dangerous for humans, though it can be toxic to
sheep and horses as a contaminant in hay.

The 'getting rid of it' is going to be very hard. The underground
part of the plant is wiry and tenacious and herbicides like Roundup
not particularly effective (it helps to bruise the horsetails before
spraying).

I've had better luck with weed-b-gon than roundup on horsetail.