A messy game, this appeals to children more than their parents! You are better off playing this outside if possible. It has a certain similarity to Jenga!
You will need:
A pudding basin to use as a mould
A knife (plastic will do)
A packet of small candy (sweets) such as M&Ms or Smartees - optional
How to play:
Start by filling the pudding basin with flour, pressing down as tightly as you can until the basin is packed really full. Put the plate on top and then invert the whole thing and lift the basin off, leaving (hopefully!) a tower of flour on the plate. Carefully balance the coin on top.
Call the children forward one at a time and let them take a tiny slice off the flour "cake", trying not to disturb the coin. You may decide to reward each successful slice with a sweet. The child whose slice make the coin topple must pay a forfeit: they have to retrieve the coin from the plate with their teeth, keeping both hands behind their back. If you warn the children in advance they will love to watch this happening!
If you are confident you can pull it off, kids love to watch you actually turn out the flour tower and place the coin on. Perhaps you can practise in advance.
There is always a child at the party who is going to get very miserable if they have to get messy, so be prepared to step in and divert attention if it happens that this child is the one who has to pay the forfeit! On the other hand there is usually also a child who is prepared to dive right in there and make too much mess, so they might need supervising too!