In this passage, Jesus is offended that people are making the Temple "a den of robbers" rather than a "house of prayer".
When I think of a robber, I think of someone who takes what they don't deserve, cheating others.
We need to make sure that the church and church staff are not doing this. No doubt the church needs money and that staff persons need to be paid for their time and effort, but how much is fair?
The point is that a church needs to understand that it is not a business. It does not exist to make a profit, but to do ministry in the most effective way possible. Ministers and church staff are to be paid, but should not be gaining wealth beyond what is appropriate. We should be using most of what is entrusted to us to do ministry, not to "rob" our congregation. We should be using tithes and offerings to call more and more people to the "house of prayer", not simply improving our own houses.
Secondly, Jesus is being praised by the children and tells the priests that God has commanded children to praise Him. Because the children are praising Jesus, He is saying that He and God are one. For God to command the children to praise Him is the same as the children praising Jesus. Jesus is God and it is appropriate for the children to praise Jesus as God.