A helmet is an essential part of a good army costume. However if you look around online, you may notice that most costume do not come with helmets. If this is the case, you will have to buy it separately, but this should not be a problem, as there are many available, including ones in child sizes. A good toy helmet should be comfortable, fairly light, and it should have an adjustable strap, to keep it in place securely.
Kids toy army helmet:
Kids toy army helmet pictured: Jacobson Hat Company Men’s Child’s Plastic Netted Army Helmet, Green, Child
Rothco Kids Camouflage Army Helmet, Woodland Camo, O/S
If the helmet is part of a Halloween costume that will only be worn once, you can probably opt for something inexpensive, but if it will be used often, you may want to pay more and get something of better quality.