The physical product might cost less than $.25 but they have to cover the cost of all the parts of the experience that are not explicitly part of the price tag, including those six-figure fireworks displays and such. They can tack it onto entrance tickets or soda prices or whatever. We aren't privy.
If it's a matter of profit justifies the act, then is it ok for me to sneak in my family of four with the cost of one admission because I think it probably cost Disney about 1/5 the price of my ticket to provide me the entertainment?
It comes down to 'they sold you one soda, with a courtesy free refill for the original recipient'. You can justify it all you want but two people sharing one soda with a free refill is not what the company intended, 90% of the time. Disney probably doesn't care. Hence the huge markup. I don't think they limit kids meals to kids. But they're like $10 so it's hardly a great deal and the fact adults can eat them is reflected in the price.