The first thing I think of when I hear Sesame Street, is the bright colors. Depending on the photos, I'd maybe use something like Birthday Monsters by Lauren Grier.
I really prefer using "non themed" kits, and these papers and a lot of the elements would be great. I feel it's easier for the photos to tell their story. That being said...I do use a lot of themed kits too though!!
I'd look for a kit that has a good primary colors. We saw a sesame street show last year and a lot of our pics had red (elmo), green (oscar) and blue (cookie, etc). The set had a lot of those same primary colors.
good luck! (I don't know the shop as well as other girls so I'll let them come up with the kits!)