I recently read the EPA's take on DEET, and there are no known side effects (at least those that occur in sizable numbers according to EPA guidelines), even for children as young as just 2 months.
They even re-evaluated DEET recently (it had originally been done many decades ago), and it still passed all of the tests with flying colors.
You too can read the report at http://www.epa.gov/pesticides/factsheets/chemicals/deet.htm
Having lived in South Florida for the vast majority of my life, I can attest to the absolute effectiveness of DEET (Flamingo. July. Sunset. - Any South Floridians will very likely know what I'm talking about). The stuff just plain works. Yes it stinks. Yes it's a bit greasy. Yes it can technically melt plastic. But it also keeps mosquitoes/noseeums/black flies/gnats at complete bay. And don't let the whole melts plastic deter you. I've used it on my skin, on my clothes, on hats, shoes, etc and have never had an adverse effect.
I use 100%. From what I understand, it's not any more effective than some lower doses, but lasts longer (and comes in a much smaller package).
BTW: Anyone have any recommendations as to an alternative container for DEET? The package that 100% DEET comes in is small, but it could be lighter.