First, I try not to sing with them. I only model for them. In the early grades I do all of my modeling in the upper register, their range. FYI, proper support is crucial! Without proper support you will damage your voice. What I can't model comfortably I play on the recorder. I try to play it on the alto recorder if I can because it resembles their timbre more closely.
In higher grades, after they are comfortable matching pitch, I explain the difference in the male voice and then make it a bit of a game. I will take a song they are very familiar with and sing it in my range. Their goal is to echo it in their range. Call and response songs and echo songs work well for this also. This helps significantly but, to be honest, I never fully get out of singing in their range. I just choose when I do it, how I do it, and never talk/sing over them.
One other thing that I do is let the kids hear voices modeled in their range. I will play some of our songs on the computer and let the students hear a female voice model it for them. That seems to be helpful because then they don't just have me as a model.