It gives them a chance to discuss their childhood with each other, find out the things that they have in common and discuss how their lives have changed over the past few years.
Refreshments are always good ideas.
It may be a good idea to have this type of program some time around midterms or finals. This would be better because the residents are feeling overwhelmed with the maturity that the educational situation that they have achieved requires and would appreciate an evening to revert back to their childhood ways if only for and hour or two.