Learn Russian: Приведите ребёнка в семь часов.

Both mean "bring", but they are used differently. Приведите comes from привести, which is used for bringing a person or animal (literally leading them on foot). If you were bringing an inanimate object, you would use принести (to carry). The prefix при- here indicates arrival, which is a spatial motion prefix, unlike the non-motion prefixes пере- and до- in your current lesson.

Why does ребёнка end in -а? Isn't the direct object supposed to be in the accusative case?

It is in the accusative case! Because ребёнок (child) is a masculine, animate noun (a living being), its accusative form copies its genitive form. Therefore, we add the -а ending.

Why is the word for hours/o'clock (часов) in the genitive plural?

When stating the time, the word час (hour) follows the standard Russian number rules. After numbers 5 and higher (like семь), you must use the genitive plural form, which is часов.

Which case follows the preposition в in the phrase в семь часов?

It uses the accusative case. To say "at a certain time", Russian uses в + the accusative form of the number. The number семь (seven) looks identical in both its nominative and accusative forms.