My DD just turned 4 and uses a Sonicare for Kids. It's great...but when we do her bottom teeth, she DEFINITELY needs to spit. I just hit the button to pause and let her spit...and start it back up again. We can usually get through the top teeth without stopping. But that's how we do it. She is learning to "hold it" in her mouth a bit better. She used to signal me to stop every 2 teeth...now we get through a whole quadrant !
Hope that helps !