Kids should do all homework by themselves. Parents should control kids doing their homework till the age of 12 more or less. Most parents will not be able to help their kids anyway once they get into a higher grade (as pathetic as it may sound), besides, there's got to be a reasonable limit. A kid should think for himself once in a teen age. My mother controlled me doing homework for the first 3 or 4 years of school, till I was 10 years old. From there on I was all by myself.
There is no homework that cannot be done by a kid alone. There are lazy kids but that's a different matter.
Then again, the schools in USA have nothing to do with scools in Europe, so I'm not aware of what the system's like. (Do not get offended, but I've heard from many sources that schools in USA are generally Very (that's a capital "V") weak).