Well, if I've learned anything from this board, it's to watch the diet. I am guessing that with 3 kids, your wife may not have the time to make sure she eats well, but every little bit helps.
I've never had kids, but I did have a poor shape last year. I started by focusing on cardio to lose fat for the first few months, with some weight lifting thrown in to start the definition. Now I've switched it, mainly doing all weights with some cardio to keep my weight in check. I decided I wanted a muscular appearance (something new for me!)
I found that doing more cardio first really showed results fast and that kept me motivated. I've gone from a size 10 to a 4 in 6 months - never thought that was possible!
I applaud her for trying to improve her health (the great body is a bonus!) and you for joining in for encouragement. Tell her to keep at it and she will be very happy with the results!!!
Have fun, Ami