Just wanted to share my experience with these as I had a nightmare trying to find a good fit for my daughter's head that would result in the helmet providing head protection rather than neck protection with increased wind resistance!
I'm sure that certain helmets will be better suited to certain head shapes, but without fail (n=3) so far the Giro Rascal has been brilliant at staying where it should, largely due to its great elasticated strap design. Bell and specialised helmets just didn't work for us. Other bonus features include the locking mechanism that means the experience of pinching skin followed by 10 mins of tears (and another 10 mins late for work) has been rendered a near obsolete experience and the 2 LED lights at the back are a great safety idea and fun for the wee ones.
Just thought I'd share my thoughts as they may save you time and money, and I'd be interested to hear how you've got on?