search for helmet. You will see hundreds of threads about the pros and cons of both. Personally, I never have worn one behind the boat, but I plan to this year.
The benefits of protection and confidence outweigh the possible negatives. A lot of people say that A helmet will increase the surface area of your head and when you hit the water the increased surface area will cause an increase in how fast you head goes from moving to standing still resulting in more concussions.
I don't think this is true. You would have to wear something the size of a sombrero to cause any measurable effect of this kind. If your helmet fits properly, the foam will do more impact absorption than increasing surface area.
In short, wear a helmet. It will make you feel more secure and maybe even be the difference between you psyching yourself out and crashing vs. letting go and landing it.
I also have a son on the way, and something like that kinda makes you realign your priorities ya know?