Good for you for looking for a helmet to go with the bike :) I have treat children in the past who have sustained head injuries from falling off a bike and hitting their head “just right” to cause a life changing injury…. One that is preventable. Do you have a local children's hospital near by? The children's hospital in Ohio measures children for helmets and the parents can buy the helmets there.
Here is some great info on fitting a helmet:
Also according to http://www.smf.org/ in order to look for a proper helmet you want to assess the 4 S's: Size, Strap, Straight and Snell.
Size: Try on several different helmets before you purchase one. The best way to gauge comfort level and fit is through comparison. The helmet should fit comfortably all the way around your head. Additional pads can be used to make it snug.
Strap: Pay attention to the chin strap. Make sure that the chin strap fits around your ear and under your chin snugly and comfortably. The helmet should not shift on your head.
Straight: Know how a helmet should fit. A helmet is meant to be worn low on the forehead, just above your eyebrows. Look into a mirror or have a friend/parent help you determine the proper fit.
Sticker: Look for Snell certification. Snell Standards are the most stringent in the world, exceeding those set by the Consumer Products Safety Commission (CPSC), American Society of Testing Materials (ASTM) and the American National Standards Institute (ANSI). Need to know which helmet is Snell certified? Browse through our helmet certification lists. (the website has a list of certified helmets).
Here are some things to keep in mind when buying a helmet:
1)Pick bright colors that are visible to drivers and other cyclists.
2) Look for a helmet that's well ventilated.
3) Make sure that the helmet has a CPSC or Snell sticker inside. These indicate that the helmet meets standards set by the Consumer Product Safety Commission (CPSC) or the Snell Memorial Foundation, a nonprofit group that tests helmet safety.
4) Make sure your child's helmet fits correctly and can be adjusted.
5) You should be able to get help finding a well-fitting helmet and adjusting it properly at any bicycle store
6) When kids wear a helmet, make sure that the straps are fastened. Also make sure they don't wear any other hat underneath it.
7) Be sure to replace any helmet made before 1999. If your child hits any surface hard while wearing a helmet, replace it — helmets lose their capacity to absorb shock after taking serious hits.
8) A few bike helmets can be used as protection for other activities, but in general, they're best suited to biking. Most helmets are made for one specific type of activity — for example, special helmets also are made for inline skating, baseball, and snowmobiling.
9) Kids should not wear any helmet when they're on a playground or climbing a tree — there is a risk of strangulation from the chin strap during these types of activities.
Yes i would buy knee and elbow pads - you can buy adjustable ones that will fit well and prevent getting hurt/scars from forming