If my child is young (5-6 years old) but is reading at a high level, do I need PAL?

You would not need the PAL Reading, but the PAL writing would be a perfect place to start along with the Poetry Memorization course.
With a very young child, you want to leave lots of time for you to read aloud to him and plenty of time to play and explore his environment. The PAL Writing will give you a strong foundation in writing.
If your child already prints, simply start with Part 2: Copy Work and Style. Instead of printing out the copy work pages, have your student copy sentences from one of their readers (or sentences you dictate from a reader). Better, have him copy the poems from the Poetry Memorization course (either from the book or from memory).
You will want to start with the Copy Work section because it also presents doing the Story Sequence orally in preparation for story writing and also presents the basics of primary grammar and IEW style. They will need that under their belt before tackling Part 3: Composition of the PAL Writing.