I have a Troxel Spirit helmet. It is ASTM/SEI-certified and it has a great dial fit. I think it's really comfortable, I haven't experienced any headaches or any discomfort. I bought a large because I have a big noggin and surprisingly enough, I was also able to dial it down small enough to fit a five year old girl perfectly. I was skeptical at first, thinking it would never go that small, but it did! Maybe she just has a giant head, too!
Here is a link to it... Troxel Spirit Helmet - Statelinetack.com
It comes in lots of pretty colors and it's inexpensive compared to other helmets. I wouldn't show in it or anything, but it's perfect for trails or schooling.