You are right that in passive voice we use was (or any other form of the verb be) and a past participle. This means we can create phrases like these:

The cake that was eaten by Joe was delicious.

 

The boy who was taken to hospital has recovered.

You will notice that both of the sentences above contain a relative clause (i.e. that was eaten/who was taken), that we can reduce to remove the pronoun and be verb form. We do this to be more economical in our expression. That means the following sentences are equally valid:

The cake eaten by Joe was delicious.

 

The boy taken to hospital has recovered.

Your mistake was to not add "who/what" to describe the boy.

Some more information on participle clauses can be found here on the BBC.

You are right that in passive voice we use was (or any other form of the verb be) and a past participle. This means we can create phrases like these:

The cake that was eaten by Joe was delicious.

The boy who was taken to hospital has recovered.

You will notice that both of the sentences above contain a relative clause (i.e. that was eaten/who was taken), that we can reduce to remove the pronoun and be verb form. We do this to be more economical in our expression. That means the following sentences are equally valid:

The cake eaten by Joe was delicious.

The boy taken to hospital has recovered.

Your mistake was to not add "who/what" to describe the boy.

Some more information on participle clauses can be found here on the BBC.