No matter the age of the child, the size of the shoe, or the style, leave ½” of space between the end of the longest toe and the end of the inside of the shoe.
Growing room is built into children's shoes. For example, if your child measures a toddler 5, the size 5 shoe, when properly fitted, will have ½” of space built in for growing. You don't necessarily need to buy a 5 1/2 for a size 5 foot. Shoes that have more than 1/2" of growing room can cause tripping. A shoe with less than 1/2" will be outgrown more quickly than average, and may even damage the foot. Remember that each brand fits differently. Use the measured size as a guide, then go up or down, if necessary, making sure there's a 1/2" of growing room. Here's an easy way to check:
The width of your thumb is about ½”. Place your thumb perpendicular to the foot at the end of the shoe. Press down to feel for the end of the longest toe. (The longest toe is not always the "big toe.") You should be able to press down without feeling any toes. Always fit to the longest foot / toe.