Climbing is an intrinsically risky sport, so it's important to buy a little insurance wherever you can, especially where your kids are concerned. Petzl's Picchu is one of the few climbing helmets on the market that's designed for itty-bitty noggins. It's also the only one that meets the safety standards for both cycling and climbing, so you can double up.
The straps on the Picchu, which looks like a shrunken version of Petzl's adult offerings, are easily adjustable for a wide range of head sizes. While it's intended for children 3-8 years old, it fit two-year-old Canaan easily, and its hard-shell design is durable enough that it should last us for the next five years or so. My favorite part about it? He actually enjoys wearing it.