Kids seem to really struggle with assurance. So many adults tell stories of repeatedly praying for salvation, just to make sure! Our Certificate of Faith was designed to be a "monument" to their confession of belief. Add their name and date, and present this to a child when they trust in Jesus for salvation, as a way to mark the occasion. 
Add a nice frame and hang it on their wall as a reminder that all that is needed is our belief in God's promise, and when we doubt, we look to the cross and remember what He did for us. If we look at our lives, it will produce doubt, but when we look to the cross (as we did at the beginning), it reassures us that Jesus loves us.