Depending on the school, S$1500-3000/month per child for private secondary education. S$500-1500 for private kindergarten. Local equivalents much cheaper
S$3500/month will get you somewhere nice to live but it won't be in the usual expat areas. For Changi Airport look at Loyang, Simei or Pasir Ris.
For domestic help S$350 + S$265 foreign worker levy + food etc. S$1000/month
Basically on the take home figures given you can have a very good "local" lifestyle and save money. You will live right up to your income and beyond if you live like an expat with a condo in town, a car and two teenagers in the International school.