"Call and gather your children."
Translation:Panggil dan kumpulkan anak-anakmu.
The given translation is incorrect because you only double the noun in plurals when it isn't obvious from the context that there are more than one of the item. As you are "gathering" children here the plural has already been expressed by "kumpulkan" and so "Panggil dan kumpulkan anakmu" is the correct answer to translate "Call and gather your children"
The lack of /me-/ at the beginning of the verb is what makes it an imperative. /mengambil/ is 'TRANSITIVE.take'. The root verb is /kambil/.
/panggil/ is the verb root for 'call'. Because only the verb root is used in the Duolingo sentence above, that makes it an imperative. To make a regular transitive verb without an imperative sense, the root verb /panggil/ would become /memanggil/.
For verb roots made into transitive verbs by adding /meN-/, /p/ transforms into [m] and /k/ transforms into [ng]. Therefore, /panggil/ becomes /memanggil/ and /kambil/ becomes /mengambil/, because of the root words from which they have been derived. The specific reason why each of those sounds becomes what it does in both words is because of a phonological process called nasalization.
Technically, you're not just adding /me-/ in front of the verb root. You're also adding a nasal feature to whatever the initial sound in the root word is /meN-/, so it changes how the initial consonant of the verb root is pronounced, or else it adds a nasal consonant in front of a vowel in cases when the first sound in the verb root is a vowel instead of a consonant).