I'm a newbie on here so please be gentle

We're going to Sunny Beach in a fortnight staying HB at the Mena Palace, first time for us in Bulgaria so obviously don't know the resort.
Basically what i want to know is what is there to do for kids, especially at night, my girls are 7 & 5 and i've read that the hotel entertainment isn't exactly family orientated so any advice on where we could take them would be much appreciated.
Also what are the waterparks like? and are they easy enough to get to? i've found the website for the one at Nessebar but can't find anything about the one in Sunny Beach. Are they worth the entrance money or are we better staying by the pool or beach?
Both my daughters love the water and swim better than me......even the 5 year old which is quite embarassing to admit

Thanks in advance
HB