I take it you've never taught a class full of 4 or 5 year-olds before.
"Right everyone, get your toothbrushes out."
"You didn't bring yours? No, you can't borrow his."
"Line up at the sink. One at a time."
"Make sure you brush for two minutes. Top, bottom, front, back, behind the teeth."
"Spit, rinse your mouth out with water. Wipe it with a paper towel."
"Right, now the next two."
I'm just imagining a class of 30. I reckon you've probably got two sinks in a reception classroom. For any other year, they'd probably have to go down the hall. You could probably have two around each sink at a time. Each one takes 2 minutes of actual brushing, plus drying, plus the time putting the toothpaste on the brush, and the natural fannying around of a little kid. 5 minutes is probably right. Five minutes multiplied by the amount of students you have to get through and the number of sinks you have, and you're looking at more like 40 minutes for everyone to brush their teeth. Now you could obviously have some of them doing an activity at the same time, but then you're not going to be able to properly supervise the activity and the teeth brushing at the same time. You might have a teaching assistant who could do it, but then any autistic, dyslexic or other SEN student will have to just struggle in the meantime. And of course during that time, the kids can't be doing anything that requires the teacher to address the whole class.
And then there's the question of stuff and hygiene. Is the school supplying the toothpaste/brush or are they bringing them from home? It's not like the school can have spares for kids to forget like they can with a pencil, unless they're going to be single-use.