Not much of a campfire but hot enough for a group of young kids (with adult supervision) to make marshmallows for S'mores !
Get a 6-8" clay flower pot and place it on top of a larger clay "plate". Fill the flower put about 1/2 way with 3/4-1" gavel. Take a heavy duty, shallow aluminum tray (or heavy duty aluminum foil) and form it into the space above the gravel. Whittle some wood shaving into the tray until it is about 1/2 full. Then add some larger dry twigs (< 1" diameter). Finally add about 4-8 charcoal briquettes.
Light the wood shaving. You may have to add some more dry twigs to get the briquettes to light.