Kids bikes come in a number of different sizes starting with balance bikes. A balance bike has no gears or pedals and your child simply trundles along using his legs - they inspire confidence because your child can always get his feet to the ground. Balance bikes such as the Ridgeback Scoot are generally suitable from age 2 up to about 5 years old although every child is different.
The next size bike up is a 12 inch wheel bike. These are normally a single gear with front and rear brakes and normally have stabilisers. Take a look at the Ridgeback MX12 or Ridgeback Minny for the girls. 12 inch bikes normally fit children between about 3 and 5 years old - you'll find out sizing estimates overlap a great deal because every child is diffferent.
14 inch wheel bikes are the next size up though you may find your child goes from a 12 inch wheel bike straight to a 16 inch wheel bike. As with a 12 inch bike you should expect to get a single speed bike with brakes and stabilisers. The Ridgeback MX14 is a good bet.
16 inch wheel bikes come next and again they will normally have a single gear, front and rear brakes and often stabilisers. Take a look at the Specialized Hotrock or for girls the Ridgeback Melody
A twenty inch wheel bike maybe the last "kids bike" you buy before an adults bike - it really depends on how quickly your child grows and how confident they are on two wheels. Some parents are keen to get their children on a very small adults bike rather than buying a 24 inch wheel bike(an adults mountain bike has a 26 inch wheel).
If you are considering a twenty inch wheel bike take a look at the Ridgeback MX20 - you get gears, a suspension fork and v-brakes! If you are considering a 24 inch wheel bike then take a look at the Specialized Hotrock A1 FS - you get a suspension fork, v-brakes and gears front and rear - This bike is also available for girls - The Hotrock Girls 24.