The best helmet is the one that fits and the one the kids will wear. New features like MIPS (anti sheer technology) is starting to creep into kids helmets but be more concerned with a good fit and comfort. Keep that chin strap nice and tight. We like adjustable retention systems to secure a good fit. Most bike helmets are single impact so once a helmet has taken a solid knock, it's done; always look for cracks and contusions regularly. Helmets are protective against skull fractures and with new sheer technology like MIPS, maybe concussion protection too. Know your child's limit and stay within it!
Kali Chakra Child
Our go to helmet for our 4-7 year old crowd, we have gone through a few of these. Loads of features and a great price. One Size: 48-54 CM, it has a dial fit closure system to secure a fit.
Where to buy: amazon | MSRP: $30 USD
Bell Sanction Full Face
At a minimum head circumference (circumference just above brow line) of 52 cm for a size small, it was one of the smallest helmets I could find for my 5 year old. Also, with a weight of 850 grams (1.8 lbs), it was also one of the lightest helmets I could find. Comparative weight helmets are almost always made from carbon fiber and can run $250 dollars plus.
Where to buy: backcountry | MSRP: $75 USD