As you'd expect, acoustic guitars come in myriad shapes and sizes. So, let's quickly break down the most common.
Generally speaking, the bigger the guitar, the bigger the sound. But obviously, larger acoustics do have some problems for kids starting out. For example, the dreadnought is arguably the most popular and common of the acoustic guitar shapes. Its large frame produces a massive tone with plenty of low-end and depth. Although this guitar may be commonplace, we would avoid it for young players. Its large size can make it uncomfortable for many youngsters. Likewise, the jumbo - typically the largest of the acoustic world - is ginormous and can be too big for little arms to get around.
Smaller sizes such as concert, grand auditorium or folk offer a more effortless playing experience. However, you will lose a little bit of tone - not something that's hugely important at this early stage.