this is the best place i have taken my kids in turkey although its not all inc its full board + ,so you can nip to the restaurants during the day and get a drink free, we only had to start paying after 9.30 pm, when restaurants started to close, it is expensive but is fab for kids,with 19 swimming pools, a water park, slides etc, the food is fantastic with different restaurants to try,loads of entertainment for the kids with various clubs, shows etc, it has its own beach although slightly shingly but if you go into olu deniz whichs is 1.5 miles away and go to the lagoon area the scenery is amazing and the beaches there are sandy, have a look at this website, it has won awards ,its truly a dream for kids.

http://www.lykiaworld.com/Cultures/en-Us/default.aspx

the other place i stayed that is slightly cheaper but although still lovely but not quite as good as lykia is letoonia but this is a 24 hr all inc have a look, has a few water slides, kids club, but private beach is shingly

http://www.letooniaresorts.com/site/fethiye/fethiye_home_english.htm

both these places can be booked in packages with different companies and diy so have a good search around for prices if you fancy 1 of these as they do fluctuate a lot , both places are out slightly on there own, but this happens quite a lot with big complexes,good luck with where ever you decide to go

I agree totally with ellie-meg, Lykia is certainly amazing, with loads of facilities and food to die for.
We, like you, need lots of activities to keep our kids - 11, 9, 6 , 6 happy and found Lykia wonderful for this.
This year we went to the Aqua Fantasy AI in Kusadasi (First Choice) which has it's own really amazing waterpark which is free of charge to hotel guests, as well as several pools, some with slides, around the hotel rooms. This was a lot cheaper that Lykia last year but has gone up since but may still be worth checking out. There's nothing wrong with the beach here despite some reports to the contrary, and the hotel is secluded but there's so much going on you needn't leave although Kusadasi's not far by bus and worth a visit.

We've just booked the Royal Wings AI at Lara Beach for June which again has it's own Aqua Park and mini fair, this is on with TC and Direct Travel.

The Hemera in Side is a fab AI hotel.It has been number 1 on TA for a few years now.There are lots of pictures on TA and also on the Barut Hotels website..It is also only 30 minutes walk from the old town and is situated on a lovely sandy beach.There are shops and bars just outside the hotel and all along the road into Side.
The Antalya area also gets slightly warmer weather than the Dalaman area.We were there 2 years ago in March and the weather was lovely.October will still be very warm.