Gear Inches are a standard measurement for comparing the relative gearing on your bike. The higher the number of gear inches the further you go for each rotation of the pedals.

Gear Inches are calculated using (D x C) / F. D is the diameter of the rear wheel, C is the number of teeth on the chain ring (front cog), and F is the number of teeth on the freewheel (rear cog).

Different combinations of tire size, chain ring and freewheel will give you the same effective gearing.

Check the table at BMXULTRA.COM and you can see that for a 20 x 1 &1/4 tyre (standard mini tire) you can have almost the same gearing running a 39/16 or a 44/18.
For younger riders (under 10) a gear inch ratio of around 52.5 is usually a good place to start, for shorter tighter tracks you can drop as low as a 50.1

Notice that due to the relative sizes of the chain ring and freewheel you get a much bigger change by replacing the freewheel than the chain ring, the change is about 3/1, that means you have to change 3 teeth on the chainring to make the same change as 1 tooth difference in the freewheel.