It is a toss up, but chances are if you made a sim heavy and even slimmed them down, you still have a good chance of heavy kids, it is already in their genetics, because if you notice, you have to have them work out every day to keep them thin, miss a few days and they are heavy again. I have a family like that, both parents on the heavy side and no matter what I did, the kids were all heavy, except one, (out of 12, mind you), so it is defenitely genetics.