Q. Can the property I owned before marriage be taken to pay for my husband's unpaid child support from his first marriage?
A. No matter when child support, or spousal support from a prior marriage for that matter, is ordered or modified your separate property is not liable for the debt and you are entitled to be reimbursed if it comes to be used to pay such debts without your consent. Family Code section 915. Community property, on the other hand, is liable for support debts.
However, there may be a right to reimbursement by the community (of which you own half) as against the other spouse's separate property if any of their separate property existed and was therefore available to pay the debt at the time the community paid the obligation.