When the Westminster Shorter Catechism was written in 1646-1648, it was designed, according to the Church of Scotland which adopted it, "to be a directory for catechising such as are of weaker capacity," in contrast the the Westminster Larger Catechism, which was, according to the same, designed to be a "a directory for those who have made some proficiency in the knowledge of the grounds of religion."
Yet, in the 19th century -- not to mention the 21st -- some catechizers found it useful to revise the Shorter Catechism for the benefit of young persons. First, is Joseph Patterson Engles (1793-1861), a ruling elder at the Scots Presbyterian Church in Philadelphia, and publishing agent for the Presbyterian Board of Publication. He designed a version of the Shorter Catechism titled Catechism for Young Children: Being an Introduction to the Shorter Catechism (1840). This work has been widely republished in modern times (and a very helpful study guide was produced about it by Jeff Kingswood, From the Lips of Little Ones: A Study in the in the Catechism for Very Little People, 2008), but it is believed that the PDF which appears on Log College Press (courtesy of Wayne Sparkman at the PCA Historical Center) is the only such scanned copy of the original work available on the internet today. The introduction is a precious word of encouragement to parents and teachers: "Emulate the spirit of the pious mother who, when asked by a witness of her patience and successful perseverance in the instruction of one of her children, 'How could you repeat that sentence to the child twenty times?' answered, 'If I had repeated it only nineteen times I should have lost my labor.'" The beginning of Engles' Catechism is also beloved by many:
Q. 1. Who made you?
A. God.
Q. 2. What else did God make?
A. God made all things.
Q. 3. Why did God make you and all things?
A. For his own glory.
Q. 4. How can you glorify God?
A. By loving him and doing what he commands.
Second, James Robert Boyd (1804-1890), a graduate of Princeton Theological Seminary, pastor, educator, author of text-books, and other works, including an exposition of the Westminster Shorter Catechism. He also wrote The Child's Book on the Westminster Shorter Catechism (1855, since republished as A Child's Guide to the Westminster Shorter Catechism, 2015). Boyd designed this work for children 12 and under, and recommends that study and memorization of this version of the catechism be undertaken for an half hour each Sabbath afternoon.
For parents who might feel that their young ones are not quite ready for the Westminster Shorter Catechism, these 19th century Presbyterian abbreviated versions may provide a suitable alternative, and while they are available in modern reprints, the introductions particularly to both works are not always included, and they are worth downloading for thoughtful consideration.