The free version of this app is pretty limited, but enough to let you try it out. I tried to get my 3 year old to do it and it seems you have to be too precise in the tracing for him to be able to do. Even I had trouble getting the shapes just right for the taste of the app. Its a little frustrating for young children, but maybe it will help them improve their skills!
An Ordinary Housewife about Bugs, shapes and robots face paint for toddler