I don't have numbers for either of your questions but my general impression is that most people who want to do research at childrens don't have much trouble finding a PI/project. As far a funding goes, I think a good majority of people who end up doing research over the summer get funded and there are several avenues to seek out funding (scholars programs, MSSRP, department funding, other grants). However, quite a few people who plan on doing research and can't get funding secured will switch over to other summer opportunities in order to get paid, so that skews things somewhat.