Yes, Denga Bay is in JB, it is just like Sai Kung (with lesser people), I saw wedding couple having banquet in one of the shop and when we walked along the coast road, we bumped into a train (electrical toy one) and we can ride on it and it brought us to the circus next to the eating area and it was a local circus, we played there with local for ferrus wheels (small one), merry go round, booth games etc....Not sure it is running year round, but fun to experience!!
However, when we leave, we have to get on a free-way like road to get a taxi to go back and was told by the Indian driver that it is a dangerours area, if we are not carrying a trolly, he won't stop for us.....But I really doubt what he said. But for saftly purpose, suggest to go back to restaurnat side to get taxi!!
If you don't plan to visit China Town of JB, then do it at airport, otherwise, everywhere is money exchange in China Town. And must drink Bak Guk Tei 肉骨茶, yummy
If you haven't been to Sentosa, you should go for at least once, your family can spend at least 2 whole day to play all activities there and they are very attractive. But at the same time, be prepared to see PRC people while you are there as there are a lot due to casino and in the shopping areas.
And check out their own website as they have promotion for room only, room + b'fast and room + b'fast + tickets, etc
If you have been to USJ, no need to go to USS, as they can't compare and almost all rides need to line up for long time. I guess, just Sentosa will cost you energy and time for two days. And if you feel like eating out, you can ride the cable car and get to the mall in the other side of the water, there are quite some good restaurants. However, if you want to visit into town further, you can try going to :
Soy street (In Chinese, it is Xiamen Street), it has some hip coffee shops and restaurants (I went there by myself during biz trip before and was during day time and I was told it will turn into bar and club at evening just like our LKF)
Quakee (The man made lake that surrounded with bar and restaurants) near Fullerton hotel!
Re hotel room at Festive, there is a double bed good for dad and mum and a bunk bed on top and there is a sofa (can turn into bed!) that can sleep one more. But if you ask hotel direct, you will have to pay for extra person and if you are OK with above arrangement, they shouldn't know. Hope it helps! Cheers