That seems to be a pretty common behavior! My son--now 2 1/2 did that too! I would not suggest giving him a tap on his thigh as that is modelling for him that a physical consequence is the way to deal with unwanted behavior. He'll try it out on his friends when they take his toys...then you'll have another behavior to deal with. (At this stage, you can begin to set boundaries and teach your child how to interact with other people, "It is not okay to hurt my body." and suggest an alternative behavior to deal with his frustration. I am guessing you are changing him, probably on the floor, with his feet pointing towards you. May I suggest turning him ninety degrees so that you are changing him with his feet pointing in a direction where you are not. That worked for my kiddo...kick the floor all you want! I also started narrating what I was doing--now we are going to change your diaper, let's open the tabs (you can have him participate), now we'll wipe you up (you can have him participate in this part too, it'll be helpful when potty training starts), okay...now let's get the fresh, clean diaper on...etc. Finally, currency works too. Give him a book, a toy, whatever, so that kicking you is not the entertainment. But, definitely try changing his direction...it stopped the behavior instantly.