Children are wired to seek protection from adults around them. Being born years before their ability to survive on the savannah, they need us to keep them safe from the dangers around them. And being adorable is how they ensure we will do so.
It is not vanity making us think our child is the cutest of all. Adults who find their offspring adorable spend more time aware of them and keeping them out of danger to survive to produce descendants. It is an evolutionary hack of our brains.
Of course, even knowing all this, I think my own is still the most adorable of all. Cognitive dissonance tells me so.