When I was baptized as a child I remember quite well that there were six steps that one must take before getting baptized. I recently looked up some articles on this subject on jw.org and the latest article says there are 9 steps. They now include field service and going to the meetings as steps. Which is odd the biblical account of the Ethiopian eunuch never mentions that he did any of those things before his baptism. That is a side point I found interesting.
The real point is that many of these steps cannot reasonably be made by a child.
- Come To Accurate Knowledge Of The Truth. While a child can easily learn the answers to many questions in a rote fashion this is not accurate knowledge. Before baptism all candidates are asked the same questions from the Organized book. It fairly easy to memorize answers to questions that you receive in advance. True accurate knowledge would require not only reading the entire bible but understanding the historical context of each book. Outside research would need to be utilized to verify facts and analyze doctrinal conclusions. Most children aren't capable of this. I would venture to guess that most children who are baptized have not even read the entire bible on their own.
- Repent From Their Former Sinful Course. This one is pretty obvious, they are kids, they don't have a previous sinful course. I can imagine it now "God I'm truly sorry that I stole that toy from my brother". Get real, children have no significant sins for which to repent and turn around.
- Make A Dedication To God In Prayer. Can a child make a dedication to God in prayer? Sure, but will he understand what such a dedication truly means. I doubt it very highly. Will a child understand that even though his prayer is a dedication to God that his baptism is something else entirely?
- Baptism. Does a child understand the life long implications of being baptized, "In the name of the father, the son and God's spirit directed organization"? Children have not fully developed and can't understand how their feelings may change over time. In many instances the reality of how certain things work in the organization are unclear to them. If they have not passed puberty it will be hard for them to understand the impact getting baptized will have on future relationships and courting. Their thinking is often clouded by desire to please their family and be a good child.
For these reasons I believe child baptisms made before the age of maturity (whatever that is for each country) are invalid and it is cruel to hold someone hostage to a choice made so early and often for the wrong reasons.