I have my mobile added already, but I bought my kids a tablet each and I can't find a way to add them to the Google Play Store account so I can download games onto the tablets. Thanks.
If I understand correctly, you want all devices under one account. All tablets and devices must be under one Google account. So whatever e-mail you use personally on your mobile, use that same e-mail/Google account for your kids' device.
Example: If your e-mail, let's say, for example is firstname.lastname@example.org, all devices must be connected to that Google e-mail account. So use the e-mail you use Google with, for your kids too.