There are two types of children's bikes - those that are designed by bike companies and those designed by toy companies. The latter are commonly known as "bike shaped objects". There is a BIG difference between the two.
I haven't got room to go into why you shouldn't buy a Bike Shaped Object here but click here to read an article which explains the dangers of buying a cheap children's bike. They're poorly made, incredibly heavy and can be unsafe. You're likely to end up with a very frustrated child and wishing you'd spent that little bit more on a proper bike.
Bike Shaped Objects tend to be associated with cartoon characters, pirates and princesses, handlebar streamers and other gimmicks. They are usually sold in supermarkets, discount stores and large warehouse stores. They always seem to be a very good price, until you have a broken bike and upset child on your hands!
The best test of whether it's a bike shaped object is to ask your child to pick it up. If they can't move it (and if you think it's heavy) then avoid!
Good quality children's bikes tend to be gender neutral, with child sized components. They are lightweight so can be moved around easily by a young child (including lifting it into a riding position when it's been left lying on the floor).