You need to see a an audiologist for folks under 18 years of age here in Washington state and probably everywhere else. I believe it’s the law.
There is probably an underlying medical issue when folks under 18 have hearing problems. So that’s where an audiologist or ENT comes in play.
Since Costco doesn’t hire audiologists normally, they hire two year technicians. They can’t provide hearing aids.
You might try buying some over-the-counter hearing aids. Jabra sells them them online.
Just make sure your grandchild is under the care of a medical professional if there is some underlying medical issue.