I would recommend Hotel Mariden, it was new last year and although most travel agents rate it 3* it is an official 4*.
Like most of the hotels, there is not a lot of things happening in the hotel but you won't have far to go to find something for them to do.
It is out the way of all the noise and you can walk to the centre in about 5-10 mins at a slow pace. You can also jump on the kids train for 2Lv.
This hotel is probaly one of the best i have ever stayed in and i have been to places such as Spain and am booked into the Dom. Republic this year but want to find a week to get back to Bulgaria because i love it so much and having spent 4 weeks in total of my summer 05 there, it feels like a second home.
Please feel free to ask questions...