I would suggest keeping your setup as is, but add in the additional router purely for the use of the children to connect to. Do not put the Superhub in modem mode.
You can set up the additional router by plugging it into the Superhub via it's WAN port (the port you would ordinarily plug into the internet on the additional router plugs into any LAN port on the Superhub) and set the gateway of the the additional router to be the Superhub. This allows you to then set up the additional router as it's own network - that is to say, it can be used to distribute DHCP addresses and have a separate SSID (wireless name) meaning you can segregate the children's devices from the rest of the home. This means you can control the device the children connect to completely independently from the superhub, which everyone else uses. You can then remove the additional router at night time, meaning none of the children's devices will have an internet connection, provided you change the Superhub's wireless password and don't tell them what it is.
so your setup would be something like this:
Do consider these likely 'get-arounds';
1) they could tether their phones to their laptops
2) they can still plug their machines directly into the superhub
There are a number of routers out there now that have built in parental controls, allowing you to restrict usage times from particular devices.