Definitely Saklikent then ! Its beautiful and a real oasis in the heat of the summer. A nice walk along the gorge, icy water to dip into, and cool shady trout restaurants to while away the afternoon. If the kids are strong swimmers they could do the rafting along the Gorge (great fun but bottom padding may be advisable !!)
Patara (approx 1 hour from Fethiye) is also good - you have some ruins to explore, a lovely little village to have lunch at and a beautiful beach that goes on and on. (Not far from Kalkan too, the Mrs would love that - nice shops and girly stuff !)
Dalyan as Mike says is good too, you can combine culture (tombs etc) with a boat trip, then an afternoon on a lovely beach.
There are lots of boat trips from Fethiye / Calis that tour the 12 islands or pop down to Gocek (lovely harbour, nice bars etc) and back. Even one with an inbuilt slide (great fun !). usually lots of kids on these trips and they nearly all carry snorkles, noodles etc.
Difficult isn't it ?! - have fun whatever you chose to do