The most important aspect of a helmet is fit. A helmet should fit snugly and stay put when you shake your head, even with the chin strap undone. Many sports stores and skate shops have trained staff who can assist you in finding the best fitting helmet for your child. I've had a good experience, both at the school and with my own kids, with Bell and Protec. These brands just seem to easily fit and properly cover most heads and they offer many different styles, colours and price ranges. Remember, if a helmet slides around on your child's head, it won't be able to do its job properly which is to protect the head in case of a fall. You could try adding thicker pads to the inside of the helmet or just buy another brand or style. Make sure the helmet is comfortable on your child's head because if it doesn't feel good, your child will have a reason to take it off and not wear it.
Helmets, generally speaking, should last about 5 years with normal wear and tear. A single impact type of helmet (most bike helmets are single impact) should be replaced after a single very hard hit to the helmet or if the inside material is breaking down or cracking. A multi impact helmet can support multiple hard hits but be aware that its performance will be reduced after each impact.
Please don’t hand down helmets to younger siblings. Every head is precious and deserves to be fitted for a new helmet.
We get a lot of questions about buying skateboards, inline skates and scooters. Personally, I don't think you need to spend an exorbitant amount of money to appropriately outfit your child for any sport. But I do believe in buying quality equipment. Quality equipment will give your child a better experience. If you buy a skateboard at a toy store, you're getting a toy. Search out a reputable store that specializes in the equipment you're looking to purchase and shop around for the best price. Also keep your eyes open for sales or discounted clearance items. For example, we scored a deep discount on some junior aggressive inline skates a few years ago because the manufacturer was discontinuing that particular model.
For skateboards, check out West49 and Jibs (as well as your local skate shop). For inline skates, check out Shop//Task and Play It Again Sports. For scooters, check out Jibs. Do yourself and your child a favor and avoid the toy stores, hardware stores, or big box stores when you're looking to buy skateboards, inline skates or scooters.