Children don’t really have that much free time than many adults, they have to go to school and all, and usually have to go to bed earlier. Beside, you only spend a small amount of your life being a child, so even assuming you’ve had access to social media since you were capable of writing, it’s likely that at the end of your life, unless you’ve suffered an early death, you’ll have spent more time as an adult on social media than as a shitposting child.