Disturbing surveillance video obtained by The Post shows a city-funded babysitter repeatedly walloping three little kids with a belt and donning a creepy Santa Claus costume to scare them — and the horrified family is now demanding answers. La’keysha Jackson, 24, began working for Bronx mother Geraldine Jaramillo a year ago via a contractor paid for by the city’s Administration for Children’s Services that provides struggling families with babysitters to help with caretaking. The single mom said she discovered the violent treatment last month when the kids’ Pennsylvania-based grandma checked a home surveillance camera in the bedroom — and was...