DEET contains plastic softeners which can damage your clothing (that's not fatal). Used combined with permetrine it can cause gene mutations which show only in the descendants according to animal testing. Ikaridine (Autan) should cause less skin irritations than DEET, but it still isn't recommended for kids under 3. Same goes for IR3535. Source: Duodecim, the (Finnish) doctors' book of medicine and other substances October 2012
IMO if something can't be considered as suitable for kids, it can't be safe for anyone. Adults just die too slow to notice the reason why.