Between 16-18 months it is very normal for kids to go through another stage of separation anxiety and coincides with moving from infancy to true toddlerhood. They have a sense of themselves as separate and apart from the adults in their life and are unsure about the new found freedom. Things like disobeying rules is to find out if what they think is true really is true as in do the rules still apply - one of the reasons we need to insist that our rules are followed at pickup no matter what age the child is. I normally just step in and talk to the child and if it gets truely out of hand I take the coat and dress them while talking to them and usually they have learned not to mess with me so I get much better cooperation. Hopefully mom or dad is taking mental notes of what I am doing/not doing, saying etc. so they can do it next time.