You will need:

The concept of a carbon footprint may be too abstract for younger children.  For this activity you can think about your environmental impact instead - so suggestions do not need to be limited to carbon-emitting ones. 

Support pupils to draw around  their feet on a piece on sugar paper.  They may find this easier drawing around a partners foot, or their shoe or you could also use a template for them to cut out.

One one side of the footprint, draw the things they do that have a harmful impact on the environment.

On the other draw the things they can do to look after the environment.

Hang these from the ceiling or display in the classroom so that children can be reminded of the good environmental choices they can make.

Follow on activity
You could follow this activity with Understand how our carbon emissions cause the Greenhouse Effect and Climate change which focuses more on the carbon part of the carbon footprint.