WiFi speeds vary greatly depending on how close you are to the router. RF interference from other devices, such as other routers in the house and neighbor’s routers, also affect WiFi performance.
Since your Internet speed is 100Mbps, the effects of parental control is also visible. With parental control turned on for all devices, some of our users reported top speeds in the range of 45-50Mbps over WiFi. You can put some devices in profiles with parental control turned off, to avoid the ceiling brought on by parental control.
You may try to select different WiFi channels for the router to see if you get any speed gains.
Since you have a high speed router already, you can piggy back the pcWRT to the existing router, so that you have two WiFi networks, one for you and one for the kids. Have the kids connect to the pcWRT with the benefits of parental controls.