There are numerous free apps available to tune your guitar, however tuning a guitar by sound is a great way to train young ears and to learn the strings.
HOW TO USE
1) Turn up your speaker to required volume.
2) Click on a large note letter and tune the appropriate string to the sound.
3) The sound will play 8 times whilst you are tuning in.
4) You can pause the sound at any time by clicking on the note again.
5) Continue until all 6 strings are in tune.