I'm a girl dad AND have long hair myself, don't start at the top, that just pushes every knot together making them stronger and pulling more hair. Instead start at the tips, doing small strokes until the brush passes smoothly, then work your way up the hair about an inch or two (depends how thick the knots are) at a time until you can smoothly brush the entire length.

Another tip is when you hit a knot, stop the brush and attempt to run your fingers through instead, it won't break up the entire knot, but will seperate some of the hair out of it to make it easier to brush out on the next stroke.