Is it OK to pass credentials to the client to allow it to upload files to Amazon S3?
The preferred way to go is to generate a presigned POST request (your backend server asks for it with your own admin credentials).
Then from client-side you upload using this pre-signed POST.
It's effectively a way to have temporary credentials, but much more easy & secure to deploy as it can be restricted to the exact file you need to have uploaded.
Here is the link if you use the Ruby SDK (https://docs.aws.amazon.com/sdk-for-ruby/v3/api/Aws/S3/PresignedPost.html) and you will easily find that for the SDK you use.
Edit: As per comment, link to official AWS doc: https://docs.aws.amazon.com/AmazonS3/latest/dev/PresignedUrlUploadObject.html