It is better if you include the full example, but if it is the quiz question I saw and tested, the answer given is correct. 

They wanted (were wanting) to see them - Ils voulaient les voir. This positioning of the object pronoun after the conjugated verb,  directly before the infinitive is the main teaching point of the lesson. "les" is the direct object of the infinitive 'voir', and goes immediately before it.

I think you may be confusing the position of object pronouns in this construct, with the position of object pronouns before the verb in 'single verb' sentences. In both structures though, the object pronouns go before the verb that they 'belong' to.

To say "They wanted them to see (it)" requires a more complex structure in French using the subjunctive as there is a change of subject for the 2 verbs. This would be directly translated to English as  "they wanted that they see (it)" - "ils voulaient qu'ils le voient". (Yes, the subjunctive form of voir here is identical spelling to the present tense form, but it is the subjunctive)

If you really did just want to say "They wanted them to (be able to) see" (as in 'have vision') - the same situation of using a subjunctive verb in the 2nd French clause would apply. The subjunctive is covered in later levels.