Learn Russian: Ребёнок сегодня капризничает: то плачет, то смеётся. - The child is acting up today: now he cries, now he laughs.

It is a paired conjunction used to show alternating actions, meaning "now this... now that..." or "first this... then that...". Here, it highlights that the child is rapidly switching back and forth between crying (плачет) and laughing (смеётся).

Why are the verbs плачет and смеётся in the imperfective aspect?

The то ..., то ... pattern almost always uses imperfective verbs. Because it describes a continuous cycle of repeating or alternating states rather than a single completed result, the imperfective aspect is the natural choice.

Is the verb капризничает only used for children?

It comes from the word for "whim" or "caprice" and means to act up, be fussy, or throw a tantrum. While it is most often used for children, you can also use it for adults who are being moody or unreasonable, and even metaphorically for machines (like a car engine acting up) or the weather.

Why does смеётся have the reflexive ending -ся? Does it mean the child is laughing at himself?

No, it just means "laughs". The dictionary form смеяться (to laugh) is one of those Russian verbs that always takes a reflexive ending (-ся or -сь) even though the action does not reflect back onto the subject. It is just a built-in part of the word you have to memorize.