Meaning:
“小孩子” means “children,” “才” means “only,” and “做选择题” means “to do multiple choice questions.” Literally translated as “only children have to do multiple choice questions,” this saying is a humorous way to imply “I am an adult who wants all” or “I am an adult left with no choices,” depending on the context. Chinese netizens often use the line as a reply when they are confronted with a difficult choice.
Example:
A: 你喜欢长头发女生还是留短发的女孩？
Nǐ xǐhuan chángtóufà nǚshēng háishì liú duǎnfà de nǚhái？
Do you like women with long hair or short hair?
B: 小孩子才做选择题。
Xiǎoháizi cái zuò xuǎnzétí。
Only children have to do multiple choice questions. (I like them all.)