The helmet should not move around on the head or slide down over the eyes when pushed or pulled. It should be snug and comfortable and sit level on the head. The front and rear straps should meet just below each ear when tightly adjusted. The chin straps should fit snugly without pinching. A helmet should fit your child for several years.
Most models come with removable fitting pads.
These can be replaced with thinner ones as your child's head grows. Buy a new helmet if it has been in a collision. Even though you can't see it, the shock-absorbing qualities of the helmet may not work. The only kind of helmet a child on a bike should wear is a bicycle helmet. Helmets such as skating or hockey helmets are designed for skating and hockey, not for biking.