Can someone ‘drop in’ on an Alexa from their phone even if they don’t have their own Alexa?

Subject says it all, really… I want to set up my elderly mother, who lives alone (apart from Alexa!) so her kids can ‘drop in’ on her; however only me has their own Alexa. Can I still enable the others so they can do so? From their phones?

Can they maybe install the Alexa app and do it via that, even if they don’t actually have an Alexa to link it to?