Re: Why can't the children lose weight?

@BrookGAmer01your game should automatically patch itself in Origin (unless you disabled the automatic updates) if you play on a PC or Mac. And I think console version of the game have something similar.🙂

But there are often bugs that EA is still working on or else they need more information about before they can fix the problem.  I do know that weight being to high has been a problem off and on in certain ages in the game, often noticeable when a Sims ages up and is noticeably a different weight category then they were in the younger age. I cannot find any bugs regarding children's weight at this time, but perhaps nobody has reported any this time around. 
So, are the kids only chubby in one household or all your households?  What about any kids in a different Save Game file?🤔

As for exercising, sometimes if Children are in an Energized mood, they can be directed to exercise by clicking on a free space of floor and either have them do pushups, sit-ups, or stretch.  
Also, children can get exercise by swimming or playing on the monkey bars.  If their weight is actually bugged, then their appearance won't go down, but they will get Motor Skill points. 🙂

If you have the Cool Kitchen Stuff Pack then there is an ice cream maker that allows your Sims to make "The Taste of Diet" ice cream that will have them loose weight in 1 or 2 servings!  Children can eat the ice cream! 🙌

I don't know if children can use the next possibility, but there is a weight reducing potion that you can purchase with Aspiration Reward Points from the Rewards Store on their Aspiration panels.  The Rewards Store button looks like a present with a bow. Click on it and scroll down to the Insta-Lean Potion that costs 750 Reward Points.  But I have never tried it on children so I don't know if you need to be at least a teen or older to take it. 🤔

Finally, if you are using any Mods or Custom Content (CC), there is a chance that some of it has become outdated, and you will need to find and remove any outdated CC item or Mod and replace it with a version that is compatible with the current version of The Sims 4 (the last game patch was earlier this month!).  I can give you instructions how to do that if you need them! 😉

Please update us on your situation! 😇