The best child helmets are light and ventilated, with impact protection equal to adult helmets. Consumer Reports covered some of them in June, 1999. Nobody recommends riding with children under one year old, whose neck structure may not be able to support a helmet. If in doubt, take helmet and child to a pediatrician and ask. Kids don't want to look like geeks, so let them pick their helmet out for best acceptance, just as you would for an adult.
Prices are low now.
There are mail order suppliers who discount the popular brands. Our Web page has other sources, mostly for helmet promotion campaigns. But we recommend buying your first helmet at a bike shop, for help with fitting. For best protection you want the helmet level and low on your head. Put thin pads in the top.