how to calculate the premium I need to pay

If the company can't tell you that before you purchase, you should find a different company.

Work out how many years you have left to live, then work out how much you can afford.
Multiply one by the other, add 20% for future inflation, deduct estimated amount in bank account at time of death, then divide this figure by the amount of children you hope to have during your life.

Once you have arrived at the resultant figure, contact your chosen Insurance Company and take out the policy that they manage to talk you into. Usually the one which will pay them the most commission.