The prior question is whether you want children to find and copy definitions of terms from their textbooks. This is the typical way teachers ask students to learn technical vocabulary, and it is nearly always ineffective in gaining control of new vocabulary.
Learning vocabulary usually takes a much richer type of instruction. Teachers must explain new words in simple terms and help students discern the boundaries of meaning from examples and nonexamples. They must help students learn how new words are used (for example, water can erode--eat away--a mountain, but families can't erode at McDonalds). Next, students need to encounter the new word in 8-12 different contexts to extricate it from its original context. Finally, students need to begin to create their own sentences.
If your question is how students comprehend word meanings from expository text, they have to learn the many ways English reveals these meanings in context. For example, writers set off new terms with appositives, e.g., "The ancient Greeks were the first people to practice democracy, or rule by the people."
Robert Sternberg has cataloged about a dozen ways astute readers can use to glean word meanings from context. These are described in his book Intelligence Applied.
If this or another answer here proves helpful in your research, you can encourage good answers by choosing one answer as the "best answer."
Cheers,
Bruce