When people worry that their child isn't going to "learn to write," two different directions surface.
Are you talking about creative writing and written story telling?
Are you talking about the mechanics of physically writing (i.e. handwriting?)
Both matter - although, IMO, one matters much more than the other.

Creative Writing
Boys and Writing
Sue Patterson
It's Important for My Kid to Learn to Write Well
Joyce Fetteroll
How Does a Child Really Learn to Write?
Patricia Zaballos
How My Kids Learned to Write
Samantha Matalone Cook
Reading and Writing Fan Fiction
Wattpad.com

Mechanics of Writing
It's good to remind yourself that good handwriting has NOTHING to do with intelligence. (Case in point: Doctors.) And as we move forward with digital technology, handwriting may be even less and less useful. Still, it's helpful to be able to read those letters from Grandma... unless she's Skyping, or FaceTiming!
But some kids do want to learn to write.
Joyfully Rejoycing | Joyce Fetteroll
I Was His Scribe
Shine | Anne Ohman
StartWrite
Odd to see this here on an unschooling page, but if your child WANTS to write, but can't form letters yet very well, this program can help you create customizable pages they can trace over.

UNSCHOOLING GUIDE:
Learning to Write

$10

Are you still wondering how unschoolers learn to write?
This THIRTY page guide will help you explore the possibilities and see writing in a completely different way from how you learned in school!
- Creating a writing-friendly home
- Ideas for supporting a home full of writers
- Learning styles and writing
- Searching for stories in the community
- Your own relationship to writing
- Ways YOU can explore and model writing more
- Deschooling and writing
- Reasons our kids may want to write
- What if they don't?
- My son's path from non-writer to a degree in Journalism
- Practical suggestions for writing
- Ideas about publishing kids' writing
- Extra Resources and Research links
- Fun and inspiring memes about writing