Answer Description
The most likely cause is that their iOS devices are simply too old to support Near Field Communication (NFC) which is required for Apple pay (and Android pay for Android devices). NFC support started in iPhone 6 devices and is generally available in all newer models. Apple pay does not require a cell or WiFi signal.
Wikipedia
Near-field communication (NFC) is a set of communication protocols that enable two electronic devices to establish communication by bringing them within 4 cm (11⁄2 in) of each otherNFC devices are used in contactless payment systems, similar to those used in credit cards and electronic ticket smart cards and allow mobile payment to replace or supplement these systems This is sometimes referred to as NFC/CTLS (contactless) or CTLS NFC NFC is used for social networking, for sharing contacts, photos, videos or files NFC-enabled devices can act as electronic identity documents and keycards NFC offers a low-speed connection with simple setup that can be used to bootstrap more capable wireless connections
Near_field_communication - Wikipedia, the free encyclopedia