Learn Russian: Ребёнок хочет пиццу.

Because it is the direct object of the sentence—the thing being wanted. In Russian, we use the accusative case for this. For feminine nouns ending in -а like пицца, the ending changes to -у.

Wait, I thought inanimate (non-living) objects don't change their endings in the accusative case. Why did пицца change?

That "no change" rule only applies to masculine and neuter inanimate nouns! For feminine nouns ending in -а or -я, the ending always changes in the accusative case, regardless of whether the object is a living creature or just a food item.

Why does the verb хочет look a bit strange?

The verb хотеть (to want) is highly irregular. Instead of keeping a consistent base, it changes slightly depending on who is doing the wanting. For "he/she/it" subjects like ребёнок, the correct form is хочет.

Is ребёнок always a masculine noun, even if the child is a little girl?

Yes! Grammatically, ребёнок is a masculine noun because it ends in a consonant (-к). If you were to add an adjective (like "good child"), you would use the masculine form of the adjective, regardless of the child's actual gender.