Annoying as this is, I would not be sending multiple reminders at this point. They 100% should have paid on the night, but chasing and chasing and chasing at this stage, just makes you look utterly desperate, and unprofessional (as irritating as that is).
I look after dogs for a living. I ask for cash, for this very reason, however, there are regular occurrences of people collecting their dog and saying "oh, I have no cash, I'll transfer it later". 95% of people do it the same day. The 5% that take longer are always affluent. Funny isn't it? I usually leave it one week before chasing by text.
Babysitting isn't a salaried job, so it's not like you were counting on this money. Suggestions of posting on FB, or threatening court, or kicking their door down, are just ridiculous and completely unprofessional. My advice would be to wait until Saturday before texting again. Keep it light :
"Hi X. Hope you're well. Just balancing my books, and I see that I haven't received payment for the babysitting service provided last weekend"
Then consider not working for them again!