I am absolutely flabbergasted at your situation!!
And so very sorry to you OP ~ Sounds like you got suckered in by this cheap, hustling family & possibly the sub par agency.
I wish you could leave, but from what you describe it doesn’t look like you can w/out any money.
I personally think the damn agency should fly you home, but from what you stated they don’t seem very loyal to their Nannies at all.
Do you have a close friend or relative who could possibly front you some money to leave, then when you get home you can sue the agency/family.
This family sounds utterly cheap and disrespectful.
Tenfold.
Shame on them for not paying for you to eat your meals w/them and for not even providing you a real bed to sleep in.
It seems they are trying to get away w/the bare minimum here.
Even if you are not successful in getting what you are owed by them - you should do EVERYTHING in your personal power to let others know exactly what happened to you AND the agency that you dealt with.
No Nanny should have to endure any of what you are describing!!
(((((( HUGS ))))))