The "Water Cycle Song" is an excellent way to teach children about the water cycle in way that they will remember. There are several songs by this name.
Have Fun Teaching provides online resources for teachers, including printable lyrics for a song about the water cycle. The song consists of four stanzas. The first gives a general overview of the water cycle, explaining that the water cycle moves water all over the Earth. The second stanza describes evaporation, the third explains condensation and the final stanza discusses the process of water returning to the Earth as precipitation. The website also provides a link to a video of a performance of the song that students can listen to and sing along with as they learn it.
A second "Water Cycle Song" can be found at Songs for Teaching. This song is a bit longer than the first, with five distinct verses that alternate with a repetitive chorus. It essentially describes the same processes as the first song; however, more attention is paid to how the water cycle is important for everyone to understand and respect. There is link where students can listen to the lyrics and sing along with the song.