I think it depends, but generally, the kids' dessert options at CS locations are different from the adult options. Most kids menus offer full meals that already include an item intended as a healthful "dessert" (e.g., grapes, yogurt). I think that cookies are almost always available as a substitute, and you can always ask (although there are no guarantees) to substitute something else from the same menu.
That's not the case everywhere, though. Some CS locations (e.g., Casey's Corner in the MK) don't have a separate kids' menu, so at those places the kids have all the same choices as an adult.
Also, given the new substitution "rules" for the DDP, if your child wants an adult dessert, you could substitute your own fountain beverage (or, incidentally, your dessert - but that would defeat the purpose in this case) for any "snack" from the same menu, and since CS desserts are typically available for a snack credit (including all those yummy cupcakes at BOG!), drinking iced water instead of a fountain beverage in exchange for an extra dessert would allow your little one to partake of an "adult" dessert along with you.
...finally, note that until WDW starts separating out adult and kid CS credits, you could technically order your child a complete meal from the adult menu at CS locations, although you'd have to ask yourself whether you felt that was ethical to do since it's against the DDP rules.
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We typically limit my daughter's junk food intake but if we are at a certain place where the desserts look extra yummy, it would be hard to say well, we get one but you don't 
