First, the law:
In NSW it is an offence for anyone to have sexual intercourse with a child under 16 years of age - consent is irrelevant. Therefore the kids themselves are both committing an offence.
Ref: Section 66C "Sexual intercourse - child between 10 and 16" NSW Crimes Act (Maximum penalty: with both parties being 15 years old, 10 years imprisonment.)
By explicitly allowing this conduct, your partner would likely be an accessory, but as you oppse it, it is unlikely that you would be.
Ref: Part 9 "Abettors and accessories" NSW Crimes Act (Maximum penalty: same as the maximum applicable to the principle offender, which in this case is 10 years.)
The only real problem I see for you, would be with concealing a serious indictable offence, which is any indictable offence with a maximum penalty of 5 years or more of imprisonment, or life imprisonment.
Ref: Section 4 "Definitions" NSW Crimes Act (contains definition for "serious indictable offence")
Ref: Section 316 "Concealing serious indictable offence" NSW Crimes Act (Maximum penalty: in these circumstances, 2 years imprisonment.)
Second, the reality:
Kids who engage in a consensual sexual relationship are hardly ever charged with any offence - in fact, I've never heard of it happening. This of course all changes if one of them makes a complaint at some point claiming that something happened without consent - in which case all of the above offences could come into play plus a lot more. Given that they have openly admitted to the sexual relationship, this is highly unlikey to ever happen.
So in short, I wouldn't worry too much about it. One thing that is certain though, is that if you try to stop it, they will simply do it elsewhere. In my opinion, you should probably leave things alone - especially if they are taking precautions. Most kids that age think they are invincible and don't take take any precautions at all. The fact that they are smart enough to do that, is at least something to be thankful for and should give you a little peace of mind. In a year's time, you won't have any say in the matter anyway.
To sum it all up - all parties here are technically breaking the law, but reality tells us that nothing will ever come of it.