One of the best ways to formulate a thesis statement, regardless of the topic, is to determine what interests you most about the piece. Are you viewing the miniseries from a cultural perspective? A parental perspective? A filmmaker's perspective? Take some time to consider what really stood out to you about the work.
After deciding what approach you want to take, brainstorm about some of the obvious points you could make. For example, if you are looking at the miniseries from a cultural point of view, some of the points you could make are that our culture has firm ideas about corporal punishment and who doles it out; definitions of corporal punishment and abuse are conflated under particular circumstances; communal approaches to correcting a child's behavior have fallen out of favor over the last few decades.
Then, do a little preliminary research on some of your points to determine if you are headed in the right direction. For example, you can search "laws about corporal punishment" and "changing attitudes about corporal punishment" and "changes in cultural norms around parenting."
Finally, decide how you will frame your discussion in the paper. Your thesis should preview the argument you will make, and make your point of view evident. So, for example, "The miniseries The Slap raises interesting points about how our culture has moved away from a collective child-rearing model to an individualistic child-rearing model" or "While 19 states allow teachers and school administrators to use corporal punishment, The Slap questions whether or if it is ever appropriate to administer such punishment on a child that isn't your own."