There are a few key assumptions common to this sort of debate, all of which are either mistaken or are not being followed to their logical conclusion:

Children are fully fledged members of society:

This is not supported by legislation or convention. Children are under the legal and moral guardianship of their parents and are therefore in posession of a limited number of freedoms. This is judged to be a fair exchange for the priviledge of being provided for, taught, guided and protected during what is becoming an increasingly longer period of dependance.
Any behaviour exhibited by a child in public is "normal":
Erm, no. Sorry. If you were to show up at the cinema butt naked and screaming, you'd be swiftly picked up and commited by the authorities. As far as your children go, you are the authorities. It is your responsibility to make sure they neither make a spectacle of themselves nor disturb the public peace. We have laws about these things, and unless you want you toddler to be subject to arrest, you'd better accept this responsibility.
Children are just inquisitive and curious, they mean no harm:
I don't know about your child, but most children are human. And humans are by birth capable of behaving like selfish, malicious, attention seeking bastards. Throwing himself on the floor kicking and screaming in the supermarket because you won't buy him the new Pokemon soda-pop ia not inquisitiveness. It's emotional blackmail, which you're free to cave in to if that is what you wish - I, however, am an innocent bystander who should not be subjected to the attendant riot.
People who complain about children being undisciplined are all sour, crabby, sterile monsters with no understanding of the joys of parenthood:
They also happen to be independant observers with a point of view which is free of your natural prejudices and emotions. Not to mention the fact that it is not only some kind of hostile minority who has to come in contact with and suffer from the raucus and unruly behaviour of loud and willful children. It's everybody who's not their parent. It's the public. A public to which you, as a member of a law-abiding society, are beholden.
Many people who have children do arrogantly expect society to be entirely geared towards making the upbringing of those children an easier matter. This is not the case. It's your own job, and not mine, to raise your children to be people who are easily and painlessly integrated into society at all ages. Just as you exercise restraint and discipline while going about your daily business (you don't urinate or defecate where you stand, you use polite language while giving or receiving a service, you pay for what you want, you restrain your voice and violent urges, etc.), so should you carry the burden of excercising that discipline on behalf of those who are too young to do so for themselves.

It does children no good to be lawless. They do not grow up free from inhibitions and complexes, or as beautiful and romantic Nature Children. They grow up belligerent and unpersuadable, self-centered and demanding, and probably meet with more heartache from dissapointed expectation than those who are taught from an early age to be considerate of others and view the world realistically - as a place where we don't always get our own way.