There are four major cellular carriers in the USA. Verizon and Sprint are CDMA networks and would not work with your phones. T-Mobile and AT&T are GSM networks, but only T-Mobile really offers no-contract plans, which is what you would want.
T-Mobile will not ship SIM cards overseas, so you will have to visit a T-Mobile store when you get here, which should not be too difficult to find in a major city. A SIM card might cost $10 or so.
There are three types of plans at T-Mobile: Simple Choice with a credit check, Simple Choice without a credit check, and pre-paid plans. I personally would get a Simple Choice plan for a month. All the credit check allows you to do is purchase phones from T-Mobile with monthly payments, which you do not need to do.
A Simple Choice plan with unlimited domestic calling, unlimited domestic/international (Australia) texting, unlimited data at 2G speeds, and 1GB of 4G/LTE high speed data per phone, is $80 for the first two lines, and then $10 each for the third and fourth lines. This will also give you 1GB of "tethering" per phone if you wish to use a computer with a phone's WiFi hotspot. Total cost would be $100 plus the cost of the SIM cards ($40 for 4?). Actually, I have never been charged for a SIM card at a store, but don't count on it.
For an additional $10 per phone you can add "Stateside International Talk" to any or all of your phones. This gives you unlimited calling to a landline overseas, or $0.20/minute calls to a cell phone. Without it a call to Australia would be over $1.50 a minute. I do not believe this is available on the Pre-Paid plans. Of course you could just use a VOIP service such as Skype or Viber instead.
Also, for $10/phone you can bump up your 4G/LTE data to 3GB. When my relatives visit they always use close to 3GB of data in a two week period by sending photos to friends and family on Facebook, Instagram or some other service. Remember, you can always change your mind and add the extra data later if you need it. This is why T-Mobile's non-contact plans are so user-friendly. And if you use a music streaming service such as Pandora while you are here, it does not count against your 4G data limit.
Should you have a tablet like an iPad, you will also get 200 MB of free data for it.
Remember that almost all phones sold in the USA are carrier-specific "locked". Hard to find an "unlocked" phone. Only buy iPhones at an Apple Store where they are truly unlocked. Do not buy phones from the Carriers' stores.
Hope this helps