Keychain and 2FA iPad Only

[Itinj24]

Both my kids only have iPads and I want to invite them into HomeKit for our home. I tried setting up keychain and 2FA on their iPads as required but it keeps asking for a trusted phone number. They don’t have iPhones and I tried using my number but it doesn’t work. Is there any other way? I thought an email account would be suitable for 2FA to work but I don’t see an option for it. Thanks.