Play with them, be happy, talk with the parents too. And realize that there are times when you can't get them to cooperate and just have to hold them down. This generally applies to the 1-3 yo's. Don't default to this as a med student though unless you have kids of your own and know when to cross the line to force them into ear and mouth exams. Also, at least in emergency and inpatient medicine, if it's at all possible to time your physical exam to after they get tylenol/motrin for the febrile kids, they'll be a lot more cooperative.