I did a great Peds ID externship during my fourth year through UC Irvine. The rotation is at Miller Children's Hospital in Long Beach. It's one of the busiest children's hospitals in Southern California so you will see a lot. Great attendings! You will learn so much in one month. Can be very demanding and you'll have to work hard, but you will have a wonderful experience when you're done. Great audition rotation to do if you are looking into UC Irvine's pediatric residency program. Obviously you should only do this during your fourth year.
If you have any questions about it, lemmee know. I really only know of rotations in the SoCal area.
Baylor College of Medicine, or more specifically, Texas Children's Hospital is a great option. I'm a resident here, and the ID team is excellent. They see all of the bread and butter stuff to manage, in addition to the truly incredible, rare infections. We had a child with visceral leishmanias. When titers were sent to CDC, they were the highest they had ever seen! We have many travelers from around the world, so we see crazy parasitic infections also. The experience here is one which will truly challenge you...our fellows work REALLY hard...best of luck.