Where will you be staying? Patong is the main big town, but the beach here is not great. You'd be better off spending beach time at Kata or Karob beaches, which are nicer.
If you're up to it, some good trips which your DCs will love are:- elephant trekking ( about 1 hour drive to get to the jungle but really nice going through the lush scenery on elephant back, and you can feed them bananas at the end if the trek) Sea canoeing up in Phang Na. Bring lots of sunscreen! Really nice and a chance to see the pillars of rocks up close. Some have monkeys that will come down to check you out. Both these are all day trips.
There are many half day snorkelling trips to see lots of colourful tropical fish, but not so good(obviously) if DCs don't like swimming in the sea. Your hotel should have a concierge with lots of leaflets and recommendations.
There is a cultural show which many people like, called Phuket Fantasea, But is very touristy. The DCS would love it though.
If you take any tuk tuks( local taxis), make sure you ask the hotel roughly how much you should be paying for your journey. Then you haggle with the driver- dont just agree with what he initially charges. Agree the price before you get in.
Night markets are in most beach areas, but the biggest is in Patong. Don't forget to haggle with the price though. Most restaurants are great, lots of fresh seafood etc. Local restaurants won't have chicken nuggets or pasta if DCs aren't adventurous, but most hotels and international restaurants will cater to western tastes, and in Patong( if the worst comes to the worst) there is a McDonalds and KFC. Also, there are many Italian restaurants around if pizza/ pasta is what you're after fit the kids.