Firstly, what do you need the cover for?
If it is to cover a mortgage or other debt that you are repaying each month, then it is decreasing term assurance that you will need, with the term to match that of the outstanding loan.
If it to cover any debts that you might have that might not be decreasing each month, e.g. credit card & overdraft, and perhaps leave a lump sum for your kid(s) then level term assurance is a better bet.
If you are wanting to leave a sum of money to cover the bills incurred by whoever you have named as guardians to look after your kid(s), then look at family income benefit.
You should have a will in which you have named the guardians for your kid(s), along with Trustees to hold any money and / or possesions on their behalf until they reach the age of 18 or 21 (you can specify in the Trust). This trust can also receive any payment from the life assurance policy.