Silvia, thank you very very much for your words of wisdom; tuning via an app is a great idea (I was thinking of a clip-on tuner) to emphasise that music comes from silence. Another aspect I must strive to implant is a sense of rhythm, so I guess singing the song to just claps first will help before playing the piece on the ukulele.
Teaching little 5yr olds must be really hard, but I guess the rewards are tremendous. Fingers crossed…