Here are few tips from the NRA's Eddie Eagle GunSafe program which teaches children what to do if they encounter a firearm:
The child should immediately stop what they are doing.
2. Don't touch!
This step is meant to help counteract the inquisitive and curious nature of children that may cause them to handle the firearm. This is the most important idea to impress upon a child.
3. Leave the area!
The child should immediately leave the area where the firearm is located.
4. Tell an adult!
After leaving the area of the firearm the child should be instructed to find a trusted adult and tell them about the firearm. While the adult may not be familiar with firearms themselves, common sense would dictate that an adult will be able to make a better, and in turn safer, decision on how to handle the situation.