You can try the following steps:
Note: I did this for Ubuntu EC2
Step 1: Login as root using the keypair
Step 2: Set up the users and their passwords according to your requirements. You can use the following commands:
sudo adduser USERNAME
sudo passwd USERNAME
Step 3: Edit the SSH configuration, which you can do using /etc/ssh/sshd_config setting
Set the following parameters as given:
- For allowing a valid user to login without the key
- If you want to allow root login without a key as well
Step 4: Run the following command to restart the ssh daemon
sudo service ssh restart
Now you will be able to login to your ec2 without the keypairs.
For CenOS instance, follow the above-mntioned steps and replace ssh with sshd in the last step.