If Wii had a post pregnancy Wii Fit, after it gathered your height and age it would ask you if you the number of babies you carried and your baby(ies)'s information (like gender birthday and name), your pre-pregnancy weight, and the weight you were when you checked in for delivery. Then it would be time for your body test.
Here it would still test your balance to see if you stand to one side or another, but the difference would be that you have to hold your baby. Standing with good balance is rather hard with a squirming 5 month old.
The little scale thingy would only have positive encouragement to say. Instead of saying, "oh, by the way, it seems like your recommended BMI/weight is toeffinlowtoeverachieve," it would say, "What a great mom you are!"
The exercises would be tailored to include the baby. For instance, in the strength training section, during the V knife exercise (lift your shoulders and feet to form a V) you would place your baby resting against your knees so that each time you came up you could give a little kiss before going back down. Lunges would involve you lifting the baby from waist height to shoulder height while you lunge. Squats would have you picking up the baby from the floor, with the helpful reminder from the "trainer" that as {your baby's name} gets heavier, it's important to pick him/her up using your legs, not your back.
The yoga section would include a modified downward facing dog, where you are reaching under the couch for the pacifier that just fell under it. The sun salutation would be "son" or "daughter" salutation- for this one you would have the baby at your feet and instead of bending over to touch your toes, you would bend to tickle his/her toes.
For aerobics, the step classes really don't need to change- those work well if you're holding your baby. You should get bonus points if you don't miss any of the steps though- squirming babies do tend to distract. The running options should have you partially bent over as though you are running after a speedy crawler. There should also be a 100 inch dash ending in a lunge and outstretched arms as if to catch an over adventurous baby.
I'm sure you could have some balance games just the same, only including increased levels such as moving the baby from one hip to the other while on the tight rope.
At the end of the training the little scale thing would calculate your calories burned and add extra to compensate for how much work it is to work out and hold the baby.
Lastly, each time you worked out, the Wii would tell you how good you look.
