When my wife and I started on the path for kids we talked about it and agreed that when the kids were full time in school she'd look for something to pull in 5-15K per year. We planned that her income would go into college and vacation funds. Assuming you all discuss and agree to a plan, you may find that he finds more purpose in the work if he has a mission (e.g. the college fund).
In our case seven years later and a couple promotions, her rejoining the work force in two years is not looking terribly necessary. But the promotions have ramped up my responsibilities. To that end, absent hiring in care, anything she would pick would have to work around the school schedule, I can't. That really limits the field some, so I no longer care about her bringing cash, and really value that home base is ably covered (not just kid care, but well, everything).
The one thing I won't support is a volunteer position that turns into essentially a full time job, with no pay. My wife is very diligent, and wouldn't leave something half done. So if she were to take on that kind of stress, I want her to be pulling in a check. Also, no "party sales" type stuff where all your profits go back into inventory and you alienate all your friends.