Step offline and hit them in the face and or collar bone. Its not to difficult, in dojo practise around here it is probably the most common response to a kick (along with direct iriminage and sokumen-iriminage). In a "real" confrontation, if someone was stupid enough to try and roundhouse me, Id be more than happy to try a tenchinage on them.
A kick like a roundhouse is generally a big long movement, you can generally see them coming a mile off. Its not too difficult to move out of the way. Isnt that the first rule of Aikido? Move.
Also, a kick is only dangerous when it has reached its full point of extension. Why would you let it do that? Seems pretty stupid to me...
" doubt Tenchinage would work from moving in. Simple anatomy- the leg is far stronger then the arm and so the potential risk to your arm far outweighs the possibility of success. Besides moving straight in seems to be rather confrontational..."
The leg may be stronger than the arm, but the arm doesnt have to go anywhere near the leg to perform Tenchinage. The concept of Irimi seems confrontational to you? Perhaps you should ask your sensei for a kicking session, I think you'd really enjoy it and would learn some interesting stuff
Edit: Forgot to add the obligatory disclaimer that I am not dissing Kicking arts. Far be it. There are some wonderful kicking techniques and strikes and of course Im sure a superb Karateka or Taekwando exponent would proceed to kick me senseless
But that isnt the way we kick in Aikido (generally Mae-Geri and Mae-Geri-Washi, in our dojo's at least) and its fairly unlikely that in a "real" confrontation you would be fighting said skilled martial artist. An Aikidoka and another MA student would hopefully know better than to waste their time in folly like this. However, things in life are rarely uncomplicated...