Thanks for inviting me out to discuss risk management of your rainwater harvesting.
As we discussed, for a rain catchment such as yours in a rural area, there is highly likely to be some pathogenic bacterial or protozoan contamination in the water at some stage – from animals and birds, and wind-blown dust. If not now, then sooner or later.
To sample it in the absence of a management programme, would really not tell us anything of value.
Since you are concerned for the health of your youngsters, I believe your best approach is to take what steps you can to prevent or minimise the risks to the water supply.
Since the main risk will likely be bacterial or protozoal infection, your suggestion to filter or treat the water is a good idea. The choice of filters is important, and it is vital to change them as recommended by the manufacturer. I recommend you look at a triple , or at least double filter system which will allow you to use filters of two to three pore sizes, the larger porosity filters being cheaper, will keep the more expensive fine filter(s) operating longer. However to get down to the smallest bacteria, reverse osmosis filters are needed, or a UV disinfection system. Be sure that you check out what the suppliers say their filters can (and cannot) handle.
Don’t worry about all the rubbish they spout about chemicals and chlorine. The real risks are microbial, and always have been.
Of course, as an EHO I can only show you examples, not make recommendations…
Beware some web sites that make outrageous claims for their filters, especially those that hint that municipal treated water is in some way unsafe. It is not.
There are also a number of improvements to the catchment system that can be used to divert contamination, or settle and flush it away.
This includes, screens, first flush diverters, inlet calming, floating outtakes and vacuum flush overflows – all of which can reduce the load of contamination getting through. An example of these products can be found here.
Sorry I could not find an Australian example! I do know you can get these products here, at least in Queensland, and I am sure the suppliers will ship anywhere. I used them when I was in the Pacific.
Some additional resources you may find useful:
I hope this is helpful.
(I am from the Government and here to help)