Boss didn't pay me what we agreed on

So today i worked 5:30-12:00am and was hired as a "waitress" at a place where alcohol is served, there is a bartender and theres food etc. comedians performed there. Since my interview, the man and i agreed on $100 for the day so $20 an hour, he knew i had no experience whatsoever and he still said he'd pay me that. He didn't ask further questions, since its inside a store where they sell pipes, bongs etc i thought idk maybe i have to be 21, so i asked if i had to be 21 to work there because I wasn't, im 19. And he googled it supposedly and said it was ok and he'd pay me cash. It is only 2 days a month.
So today, i started, i tried my best, i would take orders and give them to the cook and bartender and the bartender would give me a drink with the order info and I'd guve it to the person of who it belonged to and same with the food. 
The girlfriend of the man that hired me told me, AT THE END OF THE NIGHT that she wasn't going to pay me the same as the other workers because she thought it wasn't fair for me to get paid the same as people that had experience and aside from that i wasnt really doing anything and a lady cussed at her for not getting her drink right. She was there whenever i was taking that order and her bf ordered three things, he didnt say who they were for and im not a fucking physic so how was i supposed to know whos drinks were whos and anyways that wasnt the lady's problem her problem was what she ordered didnt have any alcohol, well thats not my fault, i dont prepare the drinks. I told her OK BUT your bf and i agreed on $100 and she said no you didn't and i said YES we did and she brought him up and he said yes we agreed on $100 but i didnt earn it etc and he was only going to pay me $50 period no discussion. So what else could i have said, not like i was going to convince them to give me $50 more or snatch it out their pocket. But i feel like this was not right at all! I feel like they took advantage of me. If i was no good for you, why didn't they just send me home? And having to be there an he n a half extra.. I have 2 kids, one which i am breastfeedingstill and being there all those hours WITH NO BREAKS was not pleasant with my boobs leaking full of milk. 
Today was their first show and with how unorganized and unprepared they were with everything i can see how unprofessional they are. 
If I wasn't struggling financially, maybe I wouldn't feel as mad and bad but, i could have been home with my kids instead of being gone for almost 7 hrs for just $50.