Unlimited from the trust point of view.
If you are talking tax, then you have to look up the ATO website and find children's tax rates and also consider the low income rebate.
I think it is about $3,600 pa before they have to pay tax now – this is on unearned income. If they have jobs and get salary etc, then they are taxed at adult rates and may earn up to $16,000 pa and no tax.