Baptists reject infant baptism (inconsistently in many cases, see this post). However, Southern Baptists seem to be increasingly okay with baptizing very young children.
In defense of this practice, I've heard it said that children are often capable of understanding very deep truths, and we shouldn't be too quick to deny that they know what they're doing when they are baptized.
That's debatable. I was thirteen when I was baptized, and I certainly didn't understand the full significance of what I was doing. In fact, the very person who said the above statement admitted that when she was baptized (at the age of about nine, I believe) she didn't fully grasp what she was doing.
But, for the sake of argument, let's say that young children are completely capable of understanding the meaning and significance of baptism (surely that's the case in at least some instances).
That still doesn't justify the practice of baptizing young children. The problem isn't whether or not they understand so much as whether or not we can accurately judge their understanding. The problem isn't that young children cannot be converted. The problem is that it's so hard for us to judge one way or the other in the case of a very young child.
How does a regenerate 5-year-old act? What about a born-again 3-year-old? How is it any different from the behavior of an unregenerate child of the same age? What does the fruit of the Spirit look like in one so young? Again, I'm not saying that young children can't be converted. I'm very confident in God's ability to draw his elect to himself at any age. I'm much less confident in our ability to discern when that drawing has taken place.
Young children are typically eager to please adults. Many (if not most) children who are baptized do so to please their parents. I don't have any statistics to back that up, but I believe most Southern Baptists, if they're honest, would have to agree.
Nearly every adult I know who was baptized before the age of eighteen, admits that he did not fully understand what he was doing when he was baptized. And yet, we still attempt to justify this practice. Perhaps we're afraid of invalidating our own baptism.
I was converted before my baptism. Yet, I didn't fully grasp what I was doing when I was baptized. I did it to please others. Had I been taught properly, I would have eventually grasped the significance of baptism on my own without any pressure from my believing family members.
Why not teach the children rather than pushing them and prodding them? Let's trust the Spirit to teach those whom He indwells. If the child is truly converted, he will be taught, not only by us, but by the Holy Spirit, and once taught, he will seek baptism in obedience to Christ. If the child is not converted, then we have no business dunking him.
I fear that, at some level, we Southern Baptists do believe in baptismal regeneration. Why else would we insist on dunking toddlers who show dubious evidence of conversion?