Draw the bottom three-quarters of an oval:
Flesh that out to make the bottom half of the strip:
Connect one of the open ends at the top to the bottom on the other side:
Now draw a straight line across the top:
Finally fill in the last edge at the back:
An advantage to this approach is that it highlights the fact that the mobius strip starts out as an ordinary strip, but has a twist in one spot.
Note I figured out these steps by making various physical mobius strips with very thin width relative to their length, and manipulating them so that they were as loop-like as possible. One of the key features is the straight line at the top. That is due to how a curve in paper looks from the side -- same as at the sides of the picture.
Finally, I echo everyone else's sentiments: you should bring strips of paper so that everyone can make a real physical mobius strip of their own. There's nothing like seeing the physical real object to support the imagination!