In my experience, students, not just children, cheat for a number of reasons.
1. They haven't had time to study because of their other/extra-curricular activities which they need good grades to participate in. For example, if the younger student enjoys skateboarding after school, their parents might not let them because they don't have their homework done and/or have decent grades on their report card.
2. They didn't choose to use their time wisely, so they didn't study. I've interviewed a number of high school aged students with poor grades and asked what they do with their time outside of school. When they have no fear of judgment being passed on their answer, they answer truthfully. To sum them up, school is boring because learning is hard, watching tv, skateboarding, participating in sports, playing music, playing video games, chatting with friends, sharing interests, just plain isn't, it is fun and enjoyable. And according to many modern-day child development theorists, these playful activities are necessary for a holistic development, but in excess can be distracting to the educational development of the student.
3. They feel the material is too hard for them, and they want to maintain the image of having good grades. Believe it or not, grades play a major part on a child's self-image and feeling of self-worth. I've heard from many that "I'm dumb, because I just don't get it. It is too hard. The teacher doesn't take the time to explain it to me.", for a number of reasons they have believed that they are stupid. At that point the only logical choice is to be apathetic and not even try. They have been there and done that, and failed before. Mastering the level of competency in a subject at the rate of other students can sometimes be a goal that is set to high for some who need extra time to synthesize the information to come to the correct answer on their own.
What it boils down to is that the student wants go get good grades, but doesn't know how to, or feels they don't have the ability level to achieve their goal. In the case of anti-cheating, it really is about teaching students early on effective study habits that work efficiently for their learning style. If we teach our students how to learn, then the information becomes fun and easy to acquire rather than a chore.
Lastly, we must realize that it is in some way a human quality to cheat. I've heard in my profession my whole life, "we live in a society of kleptography.", it is natural to take the ideas of others and build on them, so it is not too far of a stretch to say that when this ability becomes misguided it can lead to a less than an honest outcome.
I can agree with the answer given above not only children are cheating. I mean I used to know students who do the same. They told they order their assignments at the site which is called Supreme essay or something like this. I think that such help can be used only in order to proofread the assignments. But it's their personal choice I can't judge them.