This can be stated in positive form. Alternate translation: "you must change ... children in order to enter" (See: [[rc://en/ta/man/translate/figs-doublenegatives]])
Jesus uses a simile to teach the disciples that they should not be concerned with who is most important. They should be concerned with becoming humble like a child. (See: [[rc://en/ta/man/translate/figs-simile]])
The phrase "kingdom of heaven" refers to God's rule as king. This phrase is used only in the book of Matthew. If possible, keep "heaven" in your translation. Alternate translation: "enter God's kingdom" or "belong to our God in heaven when he establishes his rule on earth" (See: [[rc://en/ta/man/translate/figs-metonymy]])