The $1312 is for both kids, based on 100% physical custody. This does not include education or activity, but only for basic living. Education and activity will be on top on this but they will be based on your agreement with your hu*****and. If he does not want to pay, you can not do anything. But what kind father does not want to support his children? For your protection, I would suggest:
1. If both of you have a trunk of money, such as stock, funds, etc., open a UTMA account and deposit enough money for education of the children. The beauty of UTMA is that nobody can take them out except the children themselves.
2. Make an agreement with your hu*****and and ask him to support education. If he does not pay, sue him for breach of contract. In the agreement, specifically mention that the money will be for the education of the children so that they as third party beneficiary will also have an action if the hu*****and does not pay.
3. Rely on his consciences. What else can you do?
BTW, in California, child custody can be changed anytime after judgment. The process is extremely easy: go to the county superior court, fill a pink sheet, pay $15 and wait for two weeks. Custody is changed (provided that both of you agree to the change).