After a 5 year de facto, at a minimum she would be entitled to 50% of any increase in value over that time... If that amount is small, depending on both your financial circumstances, future needs, earning capacity etc etc, a court may order you pay a reasonably sizeable lump sum on top of that, quite possible as you have a child together... Also the possibility of making a claim to some of your superannuation...
Time frame... a claim can be made to the court within 2 years of final separation.... After that she can still make a claim but will need leave of the court to do so... Basically has to claim hardship & that her & or a child will be disadvantaged if leave is not granted. There is no time limit on this although the possibility of being granted leave to apply diminishes with time... That said, there have been cases of leave being granted many years after separation, the longest I know of was 20 years.... Again seeing a family law solicitor with all the facts & figures will give you a pretty good idea of her equitable entitlements should she pursue it & you can make a judgment call from there as to if you think it's worth drawing a line & formalising something now..
The other issue about having her & kids move back into an asset in which she still has a claim to should be obvious.... You need to go in with eyes wide open. As I said, a rental agreement is toilet paper in this scenario if she were to become aware of her rights & pursue them via the family court system.... Any unpaid rent would just form part of a joint liability, she would be in the main residence until a settlement is finalised, & if things turn ugly & she decides she is in fear of you, it's a walk to the magistrates court, a good yarn, & you won't even be able to approach her or talk except via a solicitor, & then there is the possibility the DVO will require you to move off the property all together.... So go see a solicitor, get decent solid advice, & then think long & hard about the possible regrets, that's my advice anyway..... apart from having personal experience, I have been a member of legal forums for over 10 years & I have seen many posts of people wishing they knew then what they do now & that they could turn back time