You can set it up as a separate system.  Reset the app on her device.  Factory reset the speaker.  Then restart the app on her phone and choose “Set up new system”.  

If both systems are on the same WiFi she can switch systems easily though.

I only have one wifi in the house‍♂️

hoped that I could make some kind user setup that only gave her access to a specific speaker

​@jgatie provided that option. Sonos hasn’t ever ventured into the area of ‘permissions’ for specific devices. 

I don’t know the family dynamics in your house, but in my house it was a two way street. If one of us was pranking, we would be pranked by parents and eventually there would be a truce.

Give her her own WiFi, then hook her controller and Sonos to it.

The neat thing this adds on top of the Sonos control is that you can apply filtering to her WiFi that you don’t really want on your main WiFi. I think OpenDNS still offers a free kid-safe DNS server that you could hook her WiFi to on top of anything you do local.