Here's what the CDC (see below) advises on the use of DEET-containing insect repellents:

[...] "The actual concentration of DEET varies among repellents and can be as high as 95%. Repellents with DEET concentrations of 30%-35% are quite effective, and the effect should last for about 4 hours. Rarely, children exposed to DEET have had toxic encephalopathy. The possibility of adverse reactions to DEET will be minimized if the following precautions are taken: apply repellent sparingly and only to exposed skin or clothing; avoid applying high-concentration products to the skin; do not inhale or ingest repellents or get them in the eyes; avoid applying repellents to portions of children’s hands that are likely to have contact with eyes or mouth; never use repellents on wounds or irritated skin; wash repellent-treated skin after coming indoors. If a reaction to insect repellent is suspected, wash treated skin and seek medical attention." [...]