You may not (?) be able to upgrade your adsl since nbn is available.
Everyone has to go to nbn eventually.
Were you wanting to increase speed or data limit?
What is the data limit of your current plan (peak/offpeak)?
Regarding speed. Have to maximise what you get.
Speed decreases the further from the exchange you are. Line faults can decrease speed. TPG's ADSL coverage map shows what speed you can expect at your local exchange.
Can you do a speed test from a computer connected by cable to router? (TPG support page)
Do kids use computers or tablets for school work? If computers, can they connect by cable to router or if tablets, use them close to the router.
What does the speed test show on the tablet?
Your router probably has 2 wifi bands: 2.4G and 5G. I believe the 5G has higher throughput. Each has its own wifi name so each kid could use a different one.
Your wifi speed can be affected by other wifi networks in the neighbourhood. Changing the wifi channel on the 2.4G band could also help.
TPG have prepared this: