Learn Polish: Dzieciak rozpłakał się z powodu braku deseru. - The kid burst into tears because of the lack of dessert.

The base verb is płakać (to cry). The circumfix roz- ... się adds a sudden momentum or a loss of control. So, rozpłakać się becomes a perfective verb meaning "to burst into tears" or "to start crying intensely," rather than just crying normally.

Is there a difference in tone between dzieciak and dziecko?

Yes. Dziecko is the standard, neutral word for "child" (and is grammatically neuter). Dzieciak is a colloquial noun meaning "kid" (and is grammatically masculine). It fits well here because the sentence describes a somewhat dramatic, everyday situation.

If dzieciak refers to a child, why does the verb rozpłakał use a masculine ending?

In Polish, past tense verbs agree with the grammatical gender of the subject noun. While dziecko (child) is neuter and would take the neuter ending (dziecko rozpłakało się), dzieciak (kid) is a masculine noun. Therefore, it requires the masculine past tense form rozpłakał.

Why are both braku and deseru in the genitive case?