Children, -- The word translated as "children" means "child" but in the most general sense of "offspring." Jesus  does not use it to refer specifically to children under seven, which is another term. See this article more about these words for "child."  The form could be one of address (vocative), the subject of the sentence, or the object of the sentence. 

how -- "How" is the adverb that means "how", "by any means", and "I suppose". 

hard  -- (CW)   "Hard" is the keyword here. It is an adjective that means "hard to satisfy with food", "hard to please", "discontented", "fretful", "peevish," and "difficult to explain." As an adjective, when referring to people, it means "hard to please." And when referring to things, "troublesome."

is  -- The verb "is" here is the common form of "to be" in Greek. It means to have a certain characteristic or remain in a certain condition. It also equates terms or assigns characteristics. -- When the verb "to be" appears early in the clause before the subject, the sense is more like "it is" or, in the plural, "there are." 

it -- This is from the third-person, singular form of the verb.

for them that trust in riches -- -- (OS) There is nothing in the Greek that can be translated as "for them that trust in riches" in the source we use today but it does exist in the source that the KJV translators used.

to -- This is from the infinitive form of the verb, which requires a "to" in English.

enter -- "Enter" is a word that means "go or come into" and has the double meaning of "coming into one's mind."  The tense is future and the form is the subject acting on themselves.

into  -  The word translated as "into" means "into" a place, "towards" as a direction, "in regards to" a subject, and "up to" limits in time and measure.

the -- The untranslated word is the Greek definite article.The Greek article is much closer to our demonstrative pronouns ("this", "that", "these", "those"). See this article for more. 

kingdom -- The word translated as "kingdom" can be the region, the reign, the castle or the authority of a ruler. Christ does not seem to use it to mean a physical region, so its translation as "reign" or "realm" seems more appropriate. This is especially true because the "reign" of a king means the execution of his will.

of -- This is from the genitive form of following article and noun.

missing "the"  -- (MW) The untranslated word is the Greek definite article,"the," which usually precedes a noun and, without a noun, takes the meaning of "the one." The Greek article is much closer to our demonstrative pronouns ("this," "that," "these," "those"). See this article for more. 

God! -- The word translated as "of God" means "God" and "deity." It is introduced with an article, so "the God." Jesus often uses it this way perhaps to indicate the one God as opposed to the pagan gods. It is possessive so "of the Divine"