Take some tape (duct tape or masking tape) and make a very large "V" on the ground. On one piece of tape, write "Heavenly Father". On the other, write "Satan". (Jenny - you may also write 'God' and 'Mammon' on the v if using a different scripture)
Have the children try to walk down the "V" while keeping their feet on each piece of tape. Make the "V" long and big enough for the children to have trouble keeping their feet on the tape towards the end (the big part of the "v").
When they can't do it and either fall down or choose a side to go to, tell them that that is just like our choice to choose to serve Heavenly Father or Satan.
We cannot say we want to serve God when we continue to sin and also follow Satan. We will fall or come to a point where we must make a decision.