Our kids (6 and 9 now) use re-usable plastic cups. Most of them were give-aways from various restaurants, school activities, etc. They're cheap, durable, dishwasher safe, and if they end up outside for collecting bugs or whatever, we don't care.
Maybe we're just too cheap, but my wife and I drink out of plastic cups from stadiums etc. most of the time, and the glass only comes out if we have guests generally. Finish dinner and want to take your drink with you to the garage, yard, deck, or workshop? No problem.
I'd agree with the above that stainless will get dented fairly quickly with kids. Plus the metal taste and feel isn't great for some beverages.
Surely we're not the only mustachians who drink out of cups we got for free most of the time?