I use coloured stickers near the sound-hole to help young children get started on the ukulele. These colours fit into the rainbow scale that I use on the xylophone, bells etc. The red sticker on the fretboard marks the C chord (primary-coloured chord).
There are countless resources online for all stages of ukulele learning. Here is one good example. Teachers who are interested in phonics in literacy education may note that the correct pronunciation of "ukulele" demonstrated in this video begins with a true "u" rather than "yu" as is more commonly heard. Older children may be ready to discuss the difference.