In the video you are looking at are two baby Kangaroos called Joeys out of their pouch. These two tiny cuties try to cuddle each other for body heat cos they are not suppose to be out of their pouch for months. When they are given birth to, they can be as small as a grain of rice, or as big as a bee, at 0.2 to 0.9 inches (5 to 25 millimeters) according to the; San Diego Zoo.
"When the joey is born, it is guided safely into the comfy pouch, where it gestates for another 120 to 450 days. Inside the pouch, the joey is protected and can feed by nursing from its mother's nipples till fully grown".
fully developed Joey in pouch
Although Joeys grows rapidly, they are fully matured at 14 to 20 months(females are fully grown) while its about two to four years for males to be fully grown. Female kangaroos are pregnant for 21 to 38 days, and she can give birth to up to four offspring at one time, in an extremely rare case.
Strangely, Mommy Kangaroos can have babies just few weeks apart and two varying sizes of babies nursed the same time from different nipples with different nutrients. Matured ones are Herbivorous, though they often feed on insects; they regurgitate their food like Ruminants(Cows, Sheep Goats) and could be deadly if their perimeter is bridged by both fellow Kangaroos or humans perhaps other animals.
They have been known to kill dogs and each other during fights for mates or territory. Their kicks are deadlier for humans and their daggerlike Claws could slash through a full grown adult's bowel. So they are just super tiny at birth but could be much deadlier in just two to four Years!