The disciples had failed to cast out a demon and Jesus had rebuked them for being unbelieving.    He said they were part of a generation which trusted in religious activity but did not trust the Lord (Matthew 17:14-18). Continuing the discussion with Him privately, they wanted to know why they could not heal the boy, when they had previously healed sick people and cast out demons (Matthew 10:1).
 

Tax often causes controversy.  In Jesus’ day, people paid taxes to the occupying Romans, and also a temple tax to the religious authorities (Mark 12:13-17).  Most adult males paid half a shekel (equivalent to two drachma) each year toward the upkeep of the temple.  Some (such as the Qumran sect) only paid once in a lifetime while others (the Sadducees) spoke strongly against the tax.  But it was not legally enforceable under Roman law, particularly as the shekel and half shekel coins were only used in the temple and were not common currency (the drachma was the common trading unit of money)

The disciples wanted to know which of them would be the greatest in Christ's kingdom (Matthew 18:1).  Jesus did not answer the question immediately but asked a child to be with Him as He addressed the men.  It would have been a humiliating experience for them to be told that they could not enter God’s kingdom unless they changed their attitudes to become like the small child.  Although they had been chosen to be disciples and apostles, refusal to change their hearts would prevent them from entering His kingdom.
 

Human pride is very concerned about importance. The disciples revealed their hearts by asking Jesus, "Who is the greatest in the kingdom of heaven?" (Matthew 18:1). By taking a little child as an example of dependency and trust, Jesus confronted them with the need to humble themselves (Matthew 18:4).