We took our (friends and ours) with us (2 five year olds and a 18 month old) a week or so ago.
They loved it, the boat ride over was fantastic and the place easily accessible
Couple of points:
Floor is very slippy there. Would need to hold their hand to stop them slipping over
Showers are just a trickle of running cold water which you have to use to wash off in
Providing they can handle the above then they will love it, the 'mud bath' is only 18 inches or so deep and you float as soon as you sit down. The salt bath is fantastic, as warm as a real bath and other than the smell of rotton eggs was great
Also place there to get a drink and ice cream. Dalyan is also lovely with plenty of shops and a kids play area with bars surrounding
Steve