Boy Scouts by definition ought to be boys, not girls who masquerade as boys, or girls who are gender confused.
At some point, after we’ve profoundly damaged and killed children from this great-junk science, we’ll wake up and see that transgenderism is just like the claimed epidemic of ritual sexual abuse in the 1980s and the recovered memory fad just a bit later.
The proper way to deal with a girl who insists that she is a boy is for the adults to be in charge and to hold the line, and talk about why she is upset about being a girl. A girl that wants to wear boys’ clothing can be indulged; a girl that wants to be athletic like her brothers can be encouraged. Of course certain conventions have to be observed — such as young persons born as women wear formal dresses to proms, and have young persons born men as dates, because that is just the inherent nature of the prom, and if you don’t like it, you are free not to attend — for example. In other words, children — especially very young children — have to understand that there is simply an objective biological reality they cannot change or fantasize away.