I think all the people talking about how the children could have chosen to ask to be put in the other group are kind of missing the point.
Yes, an individual child could. An opinionated, strong willed and vocal child might yell "I want to be a pirate". (DD2, looking at you there).
Other children, the shy children, the laid back children, the children most pulled by peer pressure (DD1 would be in this group) would go along with it. So they would stand still and wait to be rescued.
And it's not a problem that this time they were grouped this way. It is a problem that time after time they are grouped this way. If you look, the overall messages are remarkably consistent. And children don't have to be able to understand and process these messages to absorb them. Hence the children who have never been told that football is for boys, or boys don't wear pink, but who (seemingly out of the blue) come out with these statements.
No one jumps up and tells girls not to do physics. But by A-level hardly any do. Now that could be purely genetic (though there is no reliable evidence to support a pure genetics viewpoint), or it could be in large part the messages our children receive as they grow up about their roles in society. So if no one is telling our girls not to do physics, something more subtle is going on.
There are lots of other examples, but I won't list masses. Gender stereotyping isn't about the seriousness of one incident. If one incident was really serious, most people would disagree with it. It's the drip drip, like water on stone, over the years.