i find kids are a lot more interested and want to interact with you if you have taken the time to get to know them and play a bit beforehand.
so here are some of the main tricks i use to get a smile or a laugh.
try and capture them before they lose interest and get bored. if this does not work try the opposite and let them burn off some steam by running around. when they are feeling tired sit down with a picnic and get some more relaxed photos.
let them be free, kids don’t like being put in a spot and asked to smile. let them run around and 