Is my children's passport still valid? [closed]

This is pretty straightforward, your passport is valid until its expiry date, whether you acquired that one day before reaching the age of majority or many years beforehand.

From the Australian Government's site:

Passports for children under 16 are normally valid for five years. Child applicants aged 16 or 17 are issued with 10-year passports.

What would be the sense of issuing a 10-year passport for a 17 year old if it expired at age 18?

I'm pretty sure this is the case in any other country too.

So you will be 22 when the passport expires, which means you were 17 when it was issued (5 years) or 12 when it was issued (10 years and unlikely).

Your passport is valid until it expires in 2019. However, "valid" and "accepted" are not the same thing. Look at your picture in the passport, then look at yourself in the mirror. Or (better) ask someone you don't know all that well if the picture looks like you. This is what an immigration agent will be doing, and if the answer is "no" then it's back to Oz on the next flight.

If that's a concern, just order a new one. Renewals are usually much simpler than first applications - you fill out a much shorter form and send the old passport back.