A young child is capable of understanding that baptism is the way one joins the Church, that by doing so he is saying he wants to be a Christian, and love and serve Christ.
I was raised a Baptist, and baptized at 6 years old. Of course I didn't understand everything, or any complicated theology. My prayer life was little more than "Now I lay me down to sleep" and the Lord's Prayer, plus a halting familiarity with the 23 Psalm, but I knew that Jesus Christ loved me, died for my sins, and wanted me to be with Him in heaven, and that my first step to be a Christian was asking the Lord to forgive me and save me, then getting baptized.
It is enough a child knows he wants to belong to Jesus…that seems as good a beginning place as any… or so it seems to me.