Learn Russian: Дети любят печенье.

In Russian, печенье is treated as an uncountable mass noun, similar to how we use the word 'bread' or 'fruit' in English. It refers to the baked good in general. If you want to count individual cookies, you would use the colloquial word печенька.

Why doesn't the ending of печенье change, since it is the object of the verb?

It is actually in the accusative case, which marks the direct object. However, печенье is a neuter, inanimate noun. For inanimate neuter (and masculine) nouns, the accusative case looks exactly the same as the dictionary (nominative) form.

Is дети the normal plural form of 'child'?

Yes, but it is a highly irregular plural. The singular word for 'child' is ребёнок. The word completely changes when it becomes plural, very much like the jump from 'child' to 'children' in English.

Why does the verb end in -ят (любят) instead of -ют?

The verb любить (to love/like) is a 'Type 2' conjugation verb because its dictionary form ends in -ить. For the 'they' form (third-person plural), Type 2 verbs take the ending -ят or -ат, rather than the -ют or -ут endings used by Type 1 verbs.