It depends on how the property of others is used and whether the owners of the trademarked products and cartoon characters can successfully claim that consumers of the mural or restaurant could think that the owners of the marks/characters are the source of the art or affiliated with the restaurant, or vice versa.
Maybe the the attorneys for the agency are confident that this is "fine" because they've seen the mural's design. If they haven't seen the design, I wouldn't be so sure. Either way, your husband should have the restaurant and its ad agency indemnify him for any claims arising out of this job.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship
A quick point also is that regardless of the actual legality—whether the mural is fair use, non-infringing, etc.—the owners of any potentially trademarked matter that is included in the mural may sue or take issue with the included matter anyway. This will involve lawyers, even if your husband and the commissioning parties have done nothing wrong. On that note, I second Ms. Koslyn’s point about indemnity.
Please note that this information is for informational purposes only and is not legal advice and should not be construed as legal advice. Nothing herein is meant to create an attorney-client relationship.