Хлопчики (sing. хлопчик) is diminutive of хлопці (sing. хлопець), basically you use it to refer to younger boys, kids. Хлопець can mean boyfriend or be used in a way similar to "guy", but хлопчик is a just a small kid
The program says it's a typo when I write "й" instead of "і". It's not a typo, it's just a form of this conjunction word. Moreover, "й" is more usual in this case, because provides the sentence with better melody.