I've done this 4 times with toddler/pre-school aged kids. The first au pair did not work out. The other three did (and they were GREAT).
Au pair #1 (who didn't work out) was from a less-developed country, and had what I consider to be more "typical" au pair type experience (babysitting, a few child dev. classes). She was a poor driver, and she really didn't have the well developed "tool box" of skills to deal with young kids.
Au pairs #2 and 3 were from Germany and Austria. Both were "extraordinaire" and both had worked in a daycare setting with toddler/preschool aged kids. One was 20 and one was 25. Both were fabulous and had 1-2 years of experience working FULL TIME in a daycare. They had lots of first hand experience working with lots of types of little kids, and clearly had their bag of tricks to get kids to be cooperative, and help them grow. Also their English was VERY good (which can be really helpful with 2-3 year olds, where communication is still difficult).
Au pair #4 was from Korea. She should've/could've been "extraordinaire" - I don't know why she wasn't. She also had 2 years of experience at a daycare. She knows what it takes to care for little kids full time, and she is great. I think because of some minor language difficulties and some cultural differences, we've had to work through more issues together. Even though she knows and understands kids, it wasn't easy for her at first to have the same "in command" persona as the Europeans did. But she really has been great -- we just needed to work with her a bit more on some issues. Sometimes, when a very firm and commanding voice was needed, it wasn't coming across to the kids. Not because she didn't have the skills, but simply because she didn't have the ENGLISH. The kids picked up on the slight pauses in her speech, and (incorrectly) perceived it to be a lack of determination on her part.
Good luck. I think especially the Extraordinaires often have great experience for dealing with 2-4 year olds.