Lots of good ideas - thank you.
I'm going to sit down with them tomorrow to draw up the list - I am going to let them decide how many ticks they get for which job. Then on Monday I might buy the first prize and put it somewhere they can see it!
First prize - a bottle of nail varnish. Second prize - a small bar of chocolate.
I think I will do two prizes each time as that might help keep them going.
However, I am pretty sure that after the first prize has been given the enthusiasm will wear off and no further prizes will be needed!