In CloudShell: -
BUCKET_NAME=bucket_name
BUCKET_DIR=bucket_dir
NEW_ACL='id=12345poiuy,id=12345abcde,id=12345qwerty'
aws s3 ls --recursive $BUCKET_NAME/$BUCKET_DIR/ | \
awk '{$1=$2=$3=""; print $0}' | \
xargs -t -i \
aws s3api put-object-acl --grant-full-control $NEW_ACL --bucket $BUCKET_NAME --key {} $1