You may first write about how happy you guys were to go to your favourite uncle's house. You can express how everyone seemed to be in the right place and out of the blue, right in front of your bare eyes, two cars collided to each other.
You can mention that your blood runs cold and your entire family were terrified at what you encountered a few seconds ago. Explain what happened after that like many people crowded at the incident or screams filled the air or the residents there immediately helped the victims of the accident and called for help.
Finally, you write a conclusion and a short lesson on what you learned like to be more careful on the road.
Hope this helps.