1. In this sentence, “under the age of four” is an adjectival phrase. The phrase is describing the noun phrase “some youngsters” by giving extra information of the noun.
2. In this case, “with words” is an adverbial phrase. This phrase is functioning as an adverb of manner that gives information of how are they encouraged.
3.“of these young children” is an adjectival phrase since it provides additional information of the previous phrase, “the older siblings”, functioning as a complement.