I was at a fair recently and the stall next to me was making Mr Grass Heads, I remember doing these as a child and so promptly came home and made them with my children.
Please note I have made this look harder than it is....... it's very easy and you children should be able to most of it.
What you need is:
Some old tights
Compost
Grass Seed
Elastic Band
Empty Plastic Cup
Buttons
Glue
From the toe of the tights measure back 6-8 inches (depending on how big you want your head to be) and cut.
Put your cut off toe over your plastic cup like a hat and then gently push it down to form a pocket.
Into that pocket put a couple of children sized hands full of grass seed (or 3-4 tea spoons ) then on to the top of that add some compost, leaving enough room for you to tie the cut ends of the tights in a knot (or gather them together and wrap and elastic band round tightly up to the compost) Making sure it's secure enough the compost wont fall out.
Now you have your head you may need to shape it with your hands slightly to make it a nice round head. Not too much or your grass will move.
With the toe of the tight upwards and his tied "neck" downwards, sit him back on top of the plastic cup like a big egg.
Pinch a bit of compost and tights to form a nose and wrap an elastic band round it to hold it in place.
Super glue on two buttons to make eyes.
And water (do this over a sink as the water runs off the head to start with)
If you put your head in a nice sunny window, in less than a week your Mr Grass Head will start growing hair to look like this. (Sorry not the best photo)

It really means so much when people like my work! Thanks.