Hi Juby,
I have not stayed here but have visited lots of times with my children, there is a big kids play area, its enclosed. Lots of entertainment, kids clubs etc.. Football courts if your sis has boys. There are loads of bars/restaurants around the Club Atrium as well a their own on site, i think they have a night club too.
The beach is about a 10- 15 min slow walk maybe a bit longer in the daytime with the heat, straight out of the front gates, follow one street down and you come to the main road, accross that is the beach.
Look on this map
http://www.holidaytruths.co.uk/viewtopic.php?t=43056&highlight=map
Club Atrium is no47 and although it looks quite far away from the beach it honestly is not.
Here is another site that might give some more photos and info
http://www.clubatriummarmaris.com
As for the hassle.... you can get a bit sick of it but just say no thanks or you have already eaten, it usually does the trick.
Whatever you do though dont tell them you will eat there "tomorrow" because they wont forget and will torture you until you go back