As you may have heard, there are several different types of ukuleles. They are different from each other based on their size and tuning. Below is a chart outlining the ukulele types.
Ukulele Type Size Tuning
Soprano Smallest, 21" GCEA
Concert 23" GCEA
Tenor 26" GCEA
Baritone Biggest, 30" DGBE
Soprano will sound the brightest, concert and tenor will sound less bright, and baritone will sound the deepest. Although the tuning is the same for 3 of them, the body size will make them sound different from each other.
Most really young learners can use any size from soprano to tenor. Since baritone is the biggest it may not be suitable for children under 6 or so. Many professional players use either tenor or baritone ukuleles because of their size and tone. Of course, it can't hurt to have one of each size!
Virtual ukulele lessons are available on my website. Happy playing!