The Find My iPhone **app** (not the service) allows you to log into any icloud account. With that, all devices connected to that account having the FMiP service turned on will be listed.
If each kid has his/her own icloud account, then you'll have to log in one account at a time to track them.
If they are using your icloud account, then if FMiP service is turned on, their device should be listed when you log into your account.