Tell your husband that, without volunteers offering their time and efforts in the school, many benefits he experienced as a child and many benefits your own children are experiencing now, would not exist. Philanthropy for it's own sake is a worthy use of time.
That said, please don't take a position when you already know you will not be able to fulfill the responsibilities due to lack of time and/or other obligations. It creates a very frustrating situation for other volunteers who have to figure out how to work around the person in that position who is unavailable when they need him/her to be available.
While I'm sure it's flattering to be asked, it's just not a good match for you at this point in time and the organization would be better off pursuing another parent who has the schedule flexibility that you do not have.
I've been the volunteer/membership coordinator the past two years. They're asking me to move up into a board/officer position. I've thought about it but am afraid that because I can't really come to the school during the day I wouldn't be an effective member. Also, I miss a lot of meetings during tax season due to work. My husband said I shouldn't take a board/officer because what I do already takes up so much time and I don't get paid for it. What are your thoughts?