If you cannot log into Kiddom using the "Log In With Google" option, there may be a chance that you manually signed up with an email and password, and not through Google.
However, you can easily link your email with the log in through Google feature.
Once you are at the Log In page, click "Sign Up"
Click "Sign up with Google"
Choose the account that you entered Kiddom with
You may need to re-enter your password if you are not logged into Google
Click "Allow" when Kiddom requests access to your Google account
You will automatically be logged into your already existing account. From this point forward, you can now log in with Google!