My children are on HK passports & want to travel to china for two days with me do they need visas?

If your children have HK ID's then they will not need visas to enter China - in fact, they only need to show their ID cards to enter. But they will have to go through immigration through a different channel from you. Are they old enough to do that without you? But you can try to take them through with you (through the foreign passport holders channel) and just explain why you are doing so if they are young.