Goldfish are popular starter pets for children and a good parental choice when it comes to budget and time considerations, although you should take note that goldfish can grow to be anywhere from 6 to 12 inches and may live for as long as 10 years.
The day before you take your child to pet shop, fill clean buckets with 10 gallons (that's 16 quarts or nearly 40 liters) of water so that it has 24 to 48 hours in which to allow chlorine, lead and any other toxins to evaporate and in order that water is room temperature. Goldfish can survive in a wide range of temperatures, from above 50 to below 80 degrees Fahrenheit (that's 10 and 27 degrees Celsius respectively). It's sudden change in temperature that kills them. Add about 2 teaspoons per gallon of uniodized table salt (the big crystals, not fine stuff from your saltshaker).
What to look for
Remember to look for a lively fish when choosing, and to stay away from tanks with any obviously sick fish in them. If owner catches fish without a net, either by scooping or using his hand, that's a sign that you're dealing with a professional. Fish have a slimy protective coat (that's why they feel so slippery in your grasp) and using a net can cause damage to this surface.
If you've got a spare 10 - 20 gallon size aquarium at home, that's great. If not, any similar size waterproof container will do as long as it's got enough surface room for fish to breathe. Standard glass fishbowls are inappropriate as they are obviously too small; furthermore, they do not provide enough oxygen: every inch of fish (head and body measure) needs 20 square inches of surface (multiplying length and width of your container will give you area, so a 12" x 8" rectangular tank has a total surface area of 96 sq in., enough for a fish four inches in head/body length).