Learn Dutch: Dat is een zware straf voor een klein kind. - That is a severe punishment for a small child.

This is a great review of adjective endings after een. Straf is a de-word (de straf), so the adjective before it gets an -e (een zware straf). However, kind is a het-word (het kind). When a het-word follows een, the adjective does not get an -e (een klein kind).

Why is zware spelled with only one a when the root word is zwaar?

This is the Dutch open syllable spelling rule. When you add the -e to zwaar, the word splits into syllables as zwa-re. Because the a is now at the end of a syllable, a single a automatically makes a long vowel sound. Keeping two as would be incorrect spelling in Dutch.

Doesn't zwaar usually mean heavy?

Yes, physically it means heavy. However, it is frequently used metaphorically in Dutch to mean severe, tough, or hard. You will often see it used in contexts like een zware straf (a severe punishment) or een zware tijd (a tough time).

Since straf is a de-word, shouldn't the sentence start with instead of ?