The slip n' slide also works really well.
It could be genetic, though, unfortunately, if all of your Sims are growing up this way. I have found that some people create their males "big boned" so-to-speak and really muscular, and then when they have kids it gives them the appearance of being fat, even though their slider is average weight. I also see that some people make their females all the way skinny, but then stretch them out to look more normal. What that means is when they are an average weight they appear to be fat.
I might cheat and go into fulleditmode just to SEE where the slider is. Then you can tell for sure if they're overweight of if it's just their body structure. You don't have to change it in CAS if you don't want to.