Joint critical illness insurance
It is possible to get critical illness insurance policies which cover different people. You could have a joint policy with your spouse, but be careful with the small print; a policy might only pay out for the first of you to become critically ill. If you want cover for one of you, then you probably want it for each of you. You can also have policies that automatically cover your children when they reach a certain age. If you want this, make sure that your policy has it kicking in as and when you want it to. As always, read the small print.
Combined critical illness and life insurance policies: Often you will find critical illness policies which are bundled in with a life insurance policy. There is nothing necessarily wrong with this, even though it doesn't really follow the 'keep it simple' approach.
However, if you go for one of these, make sure that it covers exactly what you want it to. You might find that a bundled product is cheaper than two individual ones but it might mean that the bundled policy only pays out once. So, if you contracted a serious illness and then died after two months, you, or rather your dependants, would find that a bundled policy, having paid out for the illness, does not pay out on your death. Separate policies would, of course, pay out for both and are probably preferable.
Total and permanent disability cover. Many critical illness insurance policies will also include cover for 'total and permanent disability'. This pays out if you become unable to work due to permanent disability arising from any illness or injury, regardless of whether it is listed in the policy or not.
Whether or not it is a good idea to include one of these clauses is debatable. Effectively it overlaps with income protection insurance. However there are small differences. First of all, critical illness insurance pays out a lump sum whereas income protection pays an income. No doubt you would rather pay off your mortgage instead of being given the income to pay the premiums?
However, income protection insurance has one big advantage. It pays out for as long as you're off work. You don't need to be permanently disabled, just unable to work. It is therefore possible that income protection will match your requirements better than a total and permanent disability clause in a critical illness policy.