Think in terms of at least QAR 5000-6000 per term per child, multiply that by 3 for your total per school year. There are annual increases of about 10% and schools like the American School and Doha College have also started charging what they call a "capital charge", an amount in excess of the fees to cover their expenses like rent for teachers accommodation, school improvements, budget deficits, etc. In reality this "capital charge" is nothing more than a rip-off but there you have it and the Ministry of Education here knows but doesn't care.
Bottom line:
If you have a family consider your accommodation and education costs very carefully and if your employer won't cover them or help to cover them and your salary is less than QAR 15000 per month then you will suffer, especially if your wife doesn't work.