Citizens of Singapore and Spain both are exempt from a visa to Hong Kong for 90 days, Singapore citizens are exempt from a visa to Macau for up to 30 days,
Spain citizens are exempt from a visa to Macau for up to 90 days, so you four don't need a visa to Hong Kong and Macau. Singaporea citizens can visit mainland of China for up to 15 days without a visa. So you don't need a visa, but your husband, 2 sons need a tourist visa to mainland of China. It is possible for them to get tourist visa from Singapore, but I suggest they had better apply for it from Spain. It takes 4 working days for visa processing. The fee is Singapore $150 in total if they apply in Singapore. The fee is Euro 75 in total if they apply in Spain.