And I think you might be able to make an account even without having a device to install the app on
How would you do it without the app?
You can make an account here
https://www.7-eleven.com/registerYou need a phone number and email address per person, but you can use Google voice if your kids don't each have their own cell phone.
At checkout you just type in the phone number or you can pick up a 7REWARDS card in store and link it to your account.
You might have to log into your account online to check that the free slurpee coupon was added to your account before you head to the store.