Hopefully the couple of links will help you out.
Ultimately it’ll be up to yourself do source the appropriate parental path. Some internet searching may yield a better result.
As mentioned both Apple and Android have good (and free), built from the ground up, parental functionlity. Things like your kids being able to request apps to install etc. and you being able to say yes (and pay etc.) or no.
Probably most important is to start with data allowances and teaching them how to control them. Optus will charge up to $1500 a month in data top ups if they go over there limit. Andorid has again great built in data warnings and cutoffs that you can set. Apple not so much. You might consider starting with a prepaid account. This can never be exceeded (no unwanted fees) and your kid can contribute to the top up amount each month to appreciate the value of the data.
Otherwise a popular mobile option is FamilyZone. Trial the free 30 day trial?