Changi is still a functioning prison although certainly more humane than in its wretched war years. A guided tour will be well worth the money as the grounds are spread out and still under security. With your personal connection, some historical research will pay off (and prepare your emotions.) The novel and movie King Rat were inspired by the war experience but there are more up-to-date histories, especially for Australians. Both prisoners-of-war and civilian internees were kept there, made even more complicated by Singapore's competing ethnic components.
That mosaic of cultures today defines Singapore's distinctive character. The national museum traces these roots in a modern, if somewhat sanitized, fashion. http://www.nationalmuseum.sg/
You should know that the city-state is expensive by Southeast Asian standards, both for accommodation and for food. It may be the most tantalizing city in the world for gourmets because its history as a trading crossroads established diverse cuisines. For budget dining, however, the many hawkers centres thrive as food courts elevated to lick-smacking eating.
Your family's Internet research projects will help you plan ahead to get the most for your money at the dining table.

Thanks. As we have friends there too I am hoping they can help guide a bit.
Any suggestions outside of Singapore? If. Head up to Thailand ( before of after Singapore) where to go and is Kanchanaburi worth going to? Bit of beach would be nice too

Thailand is an excellent choice for a first trip to Asia. I'd spend some time in Bangkok, choose whether or not to go north to the Chiang Mai area so the kids can interact with elephants at one of the reputable elephant camps outside of CM, either the Thai Elephant Conservation Centre or the Elephant Nature Park.

For beach, you will want a place on the Gulf of Thailand side, perhaps Koh Samui, or you could go to Hua Hin, just a 2.5 hour drive from Bangkok. Read up on Kanchanburi and decide how important it is for you.

Or, instead, you could combine Bali and Singapore. Bali is an excellent family destination. For your purposes, Bali and Singapore are probably an easier combination, as there won't be so much moving from place to place.

I would definitely go to Hellfire Pass near Kanchabuburi (sp). While it has special significance for Australians (Have you read anything about Weary Dunlop?) I found it a moving experience. However, rereading your post you don't say where you are coming from so you may be fellow Aussies.