Why does the Earth tilt back and forward once a year? (Beginner)

I shared the fact that I celebrate the pagan holiday of winter solstice with some kindergardeners at the elementary school where my daughter attends second grade [I avoided the word 'pagan' like the plague]. It helped fulfill some International Bacclaureate requirement. I brought a globe and a little model sun that Chick-fil-a was giving away and explained about earth tilt and seasons and the Tropic of Capricorn [can you say 'Capricorn' boys and girls?]. But it got me thinking ... why does the earth tilt? Or more specifically why would it stop and reverse directions on a regular basis? I searched the web and got some stuff about ancient asteriod strikes. Which I can see if the Earth continued to tumble end over end. But it doesn't. So here's my ultimate question. Has anyone built a computer simulation of a primordial Earth complete with asteriod collisions that in fact models a highly periodic oscillation of 23.45 degrees from the celestial elliptic?

Becuase of the misleading way that the reason for the seasons is explained (ie. as being due to the Earth tilting towards and away from the Sun), it is a common misconception to believe that the Earth actually tilts back and forwards once a year. This would be a very strange thing to happen and I can see why you would be confused trying to come up with an explanation for it!

What's actually going on is that the tilt of the Earth always points in the same direction, but as the Earth orbits the Sun the orientation of the tilt with respect to the position of the Sun changes. There is a simple demo for this. Take a pencil and tilt it at roughly 23.5 degrees with respect to vertical. This pencil should always tilt in the same direction with respect to the walls of the room you are in. Now move the pencil in a big circle around some fixed object in the room. On one side of the object the pencil tilts towards it, while on the other side it tilts away. With respect to the walls of the room (etc) it always tilts in the same direction.

Diagram illustrating this for the Earth-Sun system: