My 5 year old daughter in junior Kindergarten, (2 grades before 1st grade) can explain what; a "force" is, a "pulley" is, and a "simple machine". She can even give examples! I was surprised when she started talking about these concepts, and when I asked where she learned aobut them, she said "Brain Pop".
Youre doing excellent work. She wrote a letter to Tim & Moby too. :)
XKCD386 about BrainPOP, v2.1.3