This is almost a straw man question. Children are the future of any society, and the future of a society or a civilization depends on the investment in children. They are the ones that will make future advances in technology and society, and they will be the custodians of it, when we are long gone.
If I did not have children, then I do not know where I would be in life, today. I believe they saved me. Not being able to go out on a whim, and the huge financial cost is something that I would sacrifice, again and again, just to receive the gift of life that they have given me.
Children are expensive, especially in today's world. In the long term, they are beneficial, however. When someone becomes older, their children are usually there to provide company and comfort. Furthermore, they carry on your family line, values, and genes into the future. In this way, you have an impact on the world, long after you are gone.
First of all, it is terrible to call children an "investment", because they are not an investment. They are people. No matter what you experience in raising them, it is worth the cost, because you gain so much understanding of human nature, selflessness, and love. Also, without children, there is no future for the world. So, having children gives you the chance to influence the world's future for the better.
Without children, our future and the whole human race wouldn't survive. They also offer you unconditional love that no amount of freedom or disposable income can replace.
Children, regardless of cost, worry, and loss of freedom, are most definitely justifiable investments. When one really considers what makes a person rich, it is not money. It is love. There is nothing like the love of a child, or the love for a child. When the end comes and one is near death, it doesn't matter how much money is in the bank. But it does matter how rich with love their life was.
If you look around the world, all of the developed countries would have negative population growth without immigration. The same is true of the urbanized developing countries. Japan and Russia, to name two, are experiencing just such a condition that will have potentially disastrous consequences down the line. Carefully planning and providing for the expense of, and loss of freedom from, having kids doesn't compare to the satisfaction of watching them grow up, and it's the best way to look to the future of mankind.
I am not going to deny that there are many expenses and turmoil that come along with raising a child. However, the good that can come will trump the bad. In most cases, a child will bring about a lot of joy to the parent. Not only does a child make the parent proud, but having children give parents a sense of self-worth. Couples who do not reproduce have a much greater chance of slipping into a mid-life crisis.
Many people would be offended if others looked at their children as "investments". Most parents do not have children for this reason. They want a child to bestow love and affection on, as well as to have a lifelong relationship with them. Even though some of these relationships turn sour, the vast majority of parents love their children dearly, and would not trade them for anything.
I believe that while raising children isn't necessarily for everyone, it can be a rewarding experience despite the emotional and financial toll it can bring on the parents. As a parent, you lose the freedom to do what you want when you want to do it, the added responsibility can be stressful, and the worry and concern for your children can be the cause of many sleepless nights, but those costs are balanced by the knowledge that you are preparing the next generation and the special moments between a parent and child (bedtime stories, craft projects, etc). These are the kind of moments we remember from our own childhoods, and to be able to pass down traditions and create those memories for our children is a benefit worth all of the effort and stress that comes with raising children.
Each person only has a limited amount of money, time, and energy. Having children requires sacrificing and committing an extraordinary amount of all of these resources. They take away a person's freedom to do things they enjoy in life. Career prospects is often diminished for parents; less time and dedication is available to devote to career development, leading to less future earnings. Having children is like voluntarily trapping one's self to a jail sentence. With all the money spent on their offspring, it is just an expense with no positive return on investment. The money is better spent on traveling to experience the world, saving for retirement, insurance policies, annuities etc. And for women, pregnancy and childbirth pure torture. 9 months of restricted diet and movements seem unbearable. Childbirth is excruciatingly painful, not to mention ruining of one's body and figure. A simple cost-benefit analysis shows cost far outweighs the limited benefits having kids provide.
Although people say that "children are our future", the world is already overpopulated enough. In this economy, the expense of a child can often be an unwise investment for individuals who are already struggling. The stress and loss of freedom that go hand-in-hand with parenting do not seem worth it to me.