"We are all born helpless. Without a parent's kindness we could not survive, much less prosper. When children grow up in constant fear, with no one to rely on, they suffer their whole lives. Because the minds of small children are very delicate, their need for kindness is particularly obvious." - H.H. The 14th Dalai Lama, How To Practice the Way to a Meaningful Life.
Children all over the world suffer from a number of causes - famine, disease, land mines, war. They are unable to have much of a say in the way their governments act, and thus are caught in the middle of the disastrous plans of a distant leader. It's well-known that women and children suffer the most in developing and warring countries, and there is a lot that needs to be done by developed nations to fix this problem.
One can argue that the most important reason (other than basic human rights) that children need to be helped is that because they are heavily influenced by their experiences and surroundings. A child raised in a compassionate, kind atmosphere is more likely to end up as a compassionate, kind adult than a child raised with no food and a gun in their hand.
UNICEF - The United Nations Children's Fund provides children in developing regions with proper food, education, medicine, and protection from violence. They teach both children and their parents the importance of nutrition, equality, and how to deal with emergences.
Coalition To Stop the Use of Child Soldiers was created and 1998 and does exactly what their name suggests - they prevent the use of children as soldiers.
International Campaign to Ban Landmines not only helps children, but everyone in a landmine ridden area.
War Child International also serves to put an end to children being forced to fight wars.