Hi tas2016! I’m currently a sophomore at UVA in the Youth & Social Innovation Major so I can surely answer your great questions.
I was in a very similar situation as you are in today. I was deciding between UVA (YSI), Cornell (Human Dev.), and also Dartmouth (Psychology). In April I visited all three and ended up choosing UVA for a lot of reasons, and can honestly say that I’m extremely happy that I did looking back.
In terms of socializing, I think UVA is ahead of Cornell since students at UVA are low-stress and very inclusive of everyone, so hanging out and talking to each other is super easy. My dorm frequently goes out in the city together and the school and student council organize plenty of bonding events. The small city of Charlottesville is more exciting than Ithaca since there are just a lot more things to do in a smaller area (restaurants, hiking, museums, pretty much something for everyone). The weather turned out to be a surprising plus too.
In terms of academics, Cornell is barely ahead in Psychology according to rankings. I’m not sure about Human Development research, but Cornell is a bit better than UVA on the research side. A trump card for my choosing of UVA though, was UVA’s internship and research connection programs which directly connect undergrads with faculty members according to research interests. UVA actually offers a course to be a Research Assistant for the Department of Psychology, the Curry School of Education, the Darden School, Medical School and others. Counselors also help you find internships in UVA and the city. See here: http://uip.coopercenter.org/?q=uip.
UVA’s academics are top-notch in general. I’ve enjoyed all of my classes in YSI since the curriculum is well-structured, classes are small, and the content is deeply interesting to me. Regarding double-majoring, my roommate is double-majoring in Psychology and something else (I honestly can’t remember lol) and is having a great time, so double-majoring is certainly possible and common.
In the end I’d say UVA is a better fit for you since UVA is a very tight-knit community with high quality academics. Cornell is a great school of course, but if you’re looking for a balanced social/academic life I’d definitely recommend UVA. Investigate more into the schools since you have plenty of time to decide.
Hope this helps, and let me know if you have questions!