As an adult, I have never had a landline in my home. I now have several pre-teen kids and none have their own cellphone. When the older ones are home alone, I leave out an old iPhone that is connected to the home wifi network; that's enough for them to call family (through Facetime) or 911. But this isn't ideal because the phone is not always charged or the younger kids hide it. Is a landline the only real solution?
What about a SIP phone? You can get a number cheaply from a cloud operator, and it can call over the internet for free.
Xiaomi 4G Kids standalone smartwatch with SOS call, SIM card and GPS Tracker. $12 each, 3 pieces. Aliexpress.
Land line is the safest bet. In the US at least, there's also a guarantee that there will always be a dialtone on a paid line even if you have a power outage in your home (assuming of course that you have an actual traditional un-powered phone plugged into the line).