I'm of two minds about this complaint.

On the one hand, I can see it being at least somewhat valid, in that the OP was told she could pay extra for the side of pasta, and once she accepted that, she was then told "Nevermind, management said no," and was not given a good reason as to why. Perhaps the waitress misspoke, perhaps management realized they didn't have the resources, whatever it was, the OP did deserve some explanation.

On the other hand, it seems like a huge fuss to make when basically all the OP was told was that, no, she couldn't break the rules and get her child free/very cheap food. In all honesty, the price difference is so small between a child's meal and a side of pasta, and leftovers can be boxed up and reheated, so I'm a tad surprised the OP had to kick up such a fuss.

As many others on here have said - be prepared! If your young'un won't eat enough to justify her own meal, either plan to eat something she can share, without major alterations/extra special requests, or bring a jar/baggie of food along for her yourself.

It's very possible that Chili's food is pre-portioned in such a way that special-making you a small side dish would waste half a serving, therefore making it more effective for the restaurant to request that you simply purchase the child's meal.

One final thought, again echoing previous posts - it does often come down to how you ask the question. When I ask to special order or substitute something at a restaurant, I ask as politely as possible and do so with no expectation that my request will be granted, as they don't have to grant it if they don't want to.