Don't think in direct and indirect objects when it comes to German. Those categories do not apply and rules you know from English will only confuse you.
In the sentence
Ich bringe meine Kinder in den Kindergarten.
the part in den Kindergarten is an adverbial of direction. The verb bringen takes a mandatory accusative object (the thing brought) and an optional dative object (the receiver).
The preposition in may take either a dative supplement, then in forms an adverbial or prepositional object of place, or it takes an accusative supplement, then it forms an adverbial or prepositional object of direction.
There are also some prepositions as wegen, which take the genitive. Wegen for example forms an adverbial of reason then.
Uh, and that article is wrong about … the person is always the dative …. A counter example, using bringen:
Der Wachtmeister bringt dem Gericht den Gefangenen.
The sergant brings the convict to the court. (The court not as a place, but as a receiver.)
Ich bringe dem Kindergarten meine Kinder.
I bring my children to the kindergarden. (Again, the kindergarten as a receiver rather than a place, which makes this sentence an eerie picture of little sacrificial lambs doomed to kindergarden hell.)
With most verbs and most times, the dative object is the receiver of the accusative object.
Also, there are about ten verbs —plus some prefixed derivatives—, including e.g. lehren, which take two accusative objects. There are two dozen common verbs which take a genitive object or accusative+genitive object and there are also a few verbs, e.g. sein and bleiben, which take the nominative!
So, forget that article altogether and get your hands on a decent German grammar book.