The 90s called with a solution…
Seriously, though, you need to define the problem more - how big is your garden, so if the phone is in the house and it rings, can she get to it in time?
This also potentially constrains other solutions - even if you get her some form of smart watch, will it be within Bluetooth range of the phone? The Bluetooth range of smartphones is around 3 - 10 metres, depending on types of walls, WiFi interference, etc…