Bear in mind that it isn't just the TSA but also US Customs officials who take an interest in what is inside the luggage of travelers, and customs officials are far more astute when it comes to picking up on illicit substances.... as are their trained sniffer dogs. Getting this into the US might be rather problematic. Not only that, but if caught entering with a federally unlawful product, the authorities would be notified, and it is quite possible that a visitor might then be barred from entry.
Not worth the risk.
They are US citizens. The only way to ban them from entering the US would be to revoke their citizenship and exile them to a foreign land. That seems rather extreme for simply trying to give your kid some medicine to prevent seizures.