The simplest hypothesis I can think of that explains this is that most things taste bad. We usually only eat the exceptions--the things that taste good. But we'll eat something that tastes bad if it gets us high.
That's certainly not true of all of the popular forms in which those (particularly alcohol and caffeine) are consumed by adults. Sure, most kids might not like black coffee or straight black tea (but plenty of adults take it with cream and sugar resp. milk and honey), but they've got very little problem with, say, Coca-Cola.
Second i'd note that virtually all foods that aren't fruits or vegetables must be 'acquired by special effort', and that a large number of them are fairly inedible or bland until you prepare or cook them. And don't children generally dislike pretty much all foods? And especially healthful foods?
[1]: http://en.wikipedia.org/wiki/Theanine#Effects
[2]: http://www.amazon.com/Now-Foods-L-Theanine-Veg-Capsules-60-C...
Also, the people that I know who drink tea, tend to drink it by the pot, whereas most coffee drinkers I know tend to drink by the cup.
When you drink tea you re-steep until the leaves lose flavor, and the other chemicals reduce in potency similarly. Coffee you re-grind for every brew.
Also, I think it's pretty easy to make all of those taste good, with the exception of tobacco (and menthol cigarettes take the worst of the bite out of that, too). Sweetened coffee, flavored dip, sweet mixers to go along with vodka, etc.