Severance pay is income and you are certainly entitled to receive child support from this pay. Until such time as your ex-husband returns to court and files for a child support modification based on a substantial change in circumstances, he will continue to owe you child support based on the current child support figure. Therefore, it makes little difference to you whether it comes from his regular pay or his severance.
This can get fairly complicated. i would suggest that you speak directly with an attorney in your area.
As mentioned, child support is usually deducted from severance pay since it is "income" from the previous employer. You should talk with the Johnson County District Court Trustee since it is charged with child support enforcement in Johnson County cases (if that is where your child support was ordered). And even if it is not deducted from the severance pay itself, you have a judgment for the amount of child support owed to you, which can be garnished from bank accounts or other income or assets. The Trustee's Office cannot offer legal advice to you, so if you have other questions, you should contact a competent family lawyer to help you through those issues.
This response does not constitute legal advice and does not create an attorney-client relationship. I am licensed to practice only in Kansas. Seek legal advice from an attorney in your state or the state in which your legal claim exists.