First and foremost, your children are relying on you for everything when they are still dependents. You have to think long and hard about the situation they would be in if you were no longer there to provide support. Of course, the same thing would apply to your surviving spouse or significant other if you have a partner.
If you do not have a sizable store of assets, you should be proactive about the implementation of an income replacement strategy. This can be done through the purchase of the appropriate level of life insurance, and term life premiums are surprisingly inexpensive for younger adults.
There is another very important consideration that has nothing to do with money. If both parents were to pass away simultaneously in an accident, who would care for the children? This question is even more profound in single parent households.
You can answer it yourself if you include the choice of a guardian when you are devising your estate plan. Should this horrible circumstance ever present itself, the court would honor your request unless the judge felt as though it was not in the best interests of the children for some reason.