The proper way to do this is to have 1 wifi network with QOS set up. You want the bandwidth capacity set to whatever your ISP is providing. Then you want to set the maximum capacity per device to be no more than half of the provided bandwidth. Not all routers have that option, but if yours does it will probably fix the issue for you and your parents.
The proper way to do this is to have 1 wifi network with QOS set up. You want the bandwidth capacity set to whatever your ISP is providing. Then you want to set the maximum capacity per device to be no more than half of the provided bandwidth. Not all routers have that option, but if yours does it will probably fix the issue for you and your parents.