"They think about freedom."
Translation:Eles pensam em liberdade.
"Pensar" is related to meditate, and "achar" to speak your mind.
Your sentence would not make sense in Portuguese. A person would probably say "achar (QUE)" and not "achar sobre"; "pensar sobre" may have the same meaning as "giving your opinion", but a bit more formal, that could be used in a question:
"O que você (acha da/pensa sobre a) exposição contínua à violência explícita na TV?" ("achar de" is much more common).