If you read the Bible, children are not accountable of their sins until they are of age, usually 12-13 years of age.  When they are old enough to know good from evil, and know what sin is.  They need to be able to know that Jesus died on the cross to save us from our sins and made it possible for us to be able to return to God when we pass from this earth.  It is all in the Holy Bible.  I'm sorry, I am not Catholic, I belong to the Church of Christ.  Our beliefs stem from the Holy Bible which was inspired by Jesus Christ.

He/she sees his/hers reflection and thinks that there is another bird there. Or, if the bird is outside, and the weather is hot, figures that maybe it is cooler inside, because he/she feels the cool of the glass, and wants to come in.