Bali is amazing. Seminyak has a lovely beach, very laid back, lots of Aussies who go for the beach lifestyle. It is a big,big surfing area. Try the local restaurants, you can choose your fish straight from the fishermans boat and they cook it for you. I would just chill out on the beach. Taxis are relatively expensive, but from Seminyak you could go down the coast - the taxi drivers will usually suggest the best locations -the Balinese are very friendly, gracious and welcoming. Avoid Kuta, although there are some water parks you may enjoy and it is the best place for clothes shopping. Hard Rock Hotel in Kuta might be nice for lunch, if you want a break from all things Balinese.
Ubud is lovely, and totally different to the coast, hundreds of temples to visit, a vast market (barter), the Monkey Forest in the centre of the town is good for children, although the monkeys can and will try to steal hats, sunglasses etc. Lots of small cafes and yoga shalas, if you ask at any they will tell you about activities for children. You can walk through the rice fields, hire bikes and take trips up to the volcanoes which is a good day out. Fabulous, cheap day spas. Most Balinese in the tourist centres speak good English and the island is very safe. Children are welcome everywhere. You can hire a car,and outside Kuta and Denpasar the roads are generally quiet, but there are no signposts. Taxis are probably the easiest way to get around, and if you use them a couple of times, you will find the driver adopts you and you get the same driver everytime. You can go further afield by boat to Lombok or fly to Yogyakarta on Java to visit Borobodur, but I wouldn't do that with younger children.
Bali has a very humid climate with frequent, heavy rain, so take light waterproofs whenever you go out.
Clearing arrivals at Denpasar can take sometime, with endless queues for visas and customs. It is worth booking your hotel transfer before you arrive, airport taxi touts and baggage handlers can be quite pushy, although that is not typical of the island.
Hope you all have a lovely time.