1)Trouble Is a Friend - Lenka
I recommend this song to those who are still beginners, as the words in the song are very simple and can be heard in everyday's life. The speed is adequate and the vocal is easy to understand.
2) Yesterday once more - The Carpenters
This one is for those that are advanced enough for something more challenging. I think the song's story is soft and suitable for children and learners of all ages. The pronunciation is pretty clear. The melody is soothing,enjoyable and not too fast to hear.
3) Secrets - One Republic
This song may require you to have some moderate skills and experiences, but I'll recommend it anyway as it's my favorite. The overall vocals aren't that hard to listen, but you may find them difficult in some parts.