PII was released in May 1986 when it was released. She was 7 when the first movie came out. With its popularity, they probably quickly penned the second movie and started filmiing the middle of the next year or the year after.
They had a good amount of special effects to add in post production and then waited to release the movie until Mem Day weekend. So she could have filmed the movie as an 8 or 9-year old.
Joey