Lacrosse ranks below soccer in concussion rates. The helmet is basically there so you don't get whacked with a stick - which I do not think could cause concussions at full full force (these stick are very light and literally will bend if it is that hard). Basically if there are concussions in lacrosse it is either from clumsiness/uncoordinated kid, someone getting "checked" and head hits ground hard (very rare - one of my sons played for years), or from a colliding "accident".
From a parent standpoint I would worry about my kids psyche more than worry about concussions. It is a hard sport because it requires speed, a lot of running and there is contact. Kids without the right athletic ability or demeanor will get exposed quickly. From an injury view, there are more bruises and boo-boos in lacrosse but injury... basketball way more dangerous.