Wills are what you need to appoint guardians for your children and arrange to manage their property for them until they're legal adults. Making a will is a critical first step in your plan.
But wills must go through the probate process, a lengthy and expensive court procedure in which a judge determines that your will is valid and supervises the distribution of your property. In most California counties it can take 6 to 18 months and cost up to 5% of your gross estate (that includes the fair market value of your house when you die, even if the bank really owns it).
If you have a house worth $1,000,000 and $100,000 worth of other assets, probate costs could be close to $24,000 and could easily be even higher. You can avoid probate costs by establishing a Living Trust.