3 sentences

1. 孩子们天真烂漫的笑容最动人。

   hái​zi men tiān​zhēn làn​màn de xiào​róng zuì dòng​rén.

   The innocent and carefree smiles of children are most touching.

   Source: Dong Chinese Dictionary

2. 他总是傻乎乎地笑着，显得非常天真。

   tā zǒng​shì shǎ​hū​hū de xiào zhe, xiǎn​de fēi​cháng tiān​zhēn.

   He's always smiling foolishly, appearing very innocent.

   Source: Dong Chinese Dictionary

3. 你如果认为每个人都会守信用，那就太天真了。

   nǐ rú​guǒ rèn​wéi měi gè rén dōu huì shǒu xìn​yòng, nà jiù tài tiān​zhēn le.

   If you think everyone will keep their word, you're being too naive.

   Source: Dong Chinese Dictionary