Are you on ADSL or NBN?
With ADSL, speed decreases the further from the exchange.
With NBN, there are different speed plans. But you are always affected by the "typical evening speed".
In either case, if the kids are on wifi devices, if they work closer to the wifi router, speed will be better.
Wifi speed also depends on congestion on the wifi "channel" being used.
You can try the speed test on TPG's support page. Use a computer connected by cable to the router. No-one else should do anything at the same time. The speed you get here has to be considered against your plan.