Hiya
I have been to both Lykia and the Belcekic Beach and I would agree with Lyn that it is a good second choice to Lykia (and about 2.5K for a fortnight cheaper :tup)
It doesn't have anywhere near the same facilities as Lykia, but it wins out on location 100 %. When I went to the BB my kids were young teens, but it was still quite safe for them to wander to the little shops during the day and pick up bits and pieces, the main beach is right out in front and whilst still shingle is better than Lykias beach.
Also unless its changed most of your fellow guests will be from the UK or Turkey so no Russians, picking up endless pieces of fruit rejecting them and putting them back for others to then eat lol. The food at the BB is almost as good as Lykia, but just the one restaurant and not quite so much choice.
You could do a lot worse.
Another two to look at might be the Olu Deniz Resort and Club Sun City not stayed at those, but they all seem to be offering something similar in terms of facilities & location.
Someone else on here also tried Aqua fantasy at Kusadasi after they had been to Lykia and a repeat visit to Lykia came up too dear and thought it was OK for what they paid, think it has it's own water park, so that might be worth researching as well.
Good Luck.
Doe