First, that is not the linchpin of libertarianism. Non aggression is. If a set of actions did not violate non aggression but involved a person not being responsible for their actions, it is still in line with libertarianism.
Once the child is born it is an independent entity and the initial contract is satisfied if the two humans wish to form an additional contract, they can.
But "responsible for your actions" doesn't actually lead to either of our positions. There are more assumptions made to get there.
A baby is not an independent person.