When purchasing a helmet for your child you still need to follow the same general rule regarding sizing to ensure they are well protected. You can allow for a slightly larger fit as it could turn into a costly exercise upgrading to a new size as your child grows but please always keep the importance of helmet fitment in mind.
Before you let your child ride have a good look at how the helmet is sitting, check if the chin strap is secure and would not slip off during an accident as wearing a loose fitting helmet the risk of this will be higher than a correctly fitting helmet. When the chin strap is tightened adequately you should just be able to fit one finger between the strap under the riders chin.
The most common mistake people make with buying helmets for their kids is trying to keep them in a youth sized helmet for too long. The reason for this is the idea of saving money as youth helmets are cheaper than adult sizes, the reason for the price difference is a helmet changes completely when you jump from a kids to adults. The larger kids and smaller adults sized helmets often cross over and some kids will fit into both, if your child can fit into both an adults and kids sized helmet always choose the adults size!
The reason for this is simply because an adult helmet is safer, kids helmets are made with a significantly thinner EPS liner to reduce weight as most younger children would struggle to bear the weight of a full sized helmet on their head especially doing something like motocross with bumps, jumps and other tricky obstacles they have to navigate. Helmet companies can get away with a thinner EPS liner as most kids won’t be going as fast as an adult on a track so the forces experienced during a crash are much lower. Having said that, the sooner your child can get into a full sized helmet the safer they will be so always keep this in mind!