Your example doesn't make a connection between the present perfect and a past reference:
I have just bought a car which I wanted to buy as a child.
The sentence contains a main clause which can stand on its own: I have just bought a car, in which the present perfect is correctly used, and the relative clause: which I wanted to buy as a child, which is used only to identify the noun that precedes it.
Obviously, the absence of any temporal relation between the two sentences would have been more evident if you had written the example as two separate sentences:
I have just bought a car. This is the car I wanted to buy as a child.