Right. While there's no law requiring that people be "nice," the OP stated:
While I suppose that reasonable minds may differ, and acknowledge that stepping on someone's foot may be accidental, I would suggest that pushing is far from any definition of "accident" that I'm familiar with. Classic battery, period. And if there's any question as to whether the pushing was intentional or not, that's a question for the jury. The resulting harm - if there's any at all - need not be intentional. Either way, like you suggested, guests should respond appropriately to this, and there's ample PC for the arrest.
What happens when someone is injured because someone is being reckless?
I argue that the Brazilians were being reckless. Whether they intended to harm the OP's girlfriend or not, they did.
I would have insisted the police be called.
If I could give anyone reading this any advice, ALWAYS file a police report if you are ever pushed, shoved, or harm in any way. If anyone steals anything from you, ALWAYS file a police report. In many cites, you can do this over the phone via 311. I bet that WDW has security offices where you can fill out police reports.
It is FREE, relatively easy, and takes about 15 minutes to file a report. The police send you a letter the next week informing you that the report is on file. In many cases, you get a case number for the report on the phone (I know this because my property has been subject to a lot of vandalism over the years, with high schoolers breaking windows and harming my livestock, so I know how to fill out these reports well).
Why does this matter? Because police reports are recording and are chronicled. If no one is filling out police reports on the Brazilians, then Disney can claim there is no problem with them. However, if there are consistently police reports being filed on the Brazilians, then corporate is going to take notice.
No police report = plausible deniability of the problem.
Filing police reports = Disney will have to take action to avoid future liability.
Just imagine one day a Brazilian group seriously injures someone. If there was a paper trail of police reports on the Brazilians' activities through the years, then Disney would be facing a jury situation where police report after police report for years telegraphed to them that they needed to address the Brazilians' behavior. If they didn't try to curb it, then they would be in big trouble in front of a jury when the Brazilians finally harm someone big time.