Answer
$56$ orders.
Work Step by Step
It is necessary to choose three of the eight positions for boys.
Combination of 8 distinct childern taken 3 at a time:
$_8C_3=\frac{8!}{3!(8-3)!}=\frac{8!}{3!\times5!}=\frac{8\times7\times6\times5\times4\times3\times2\times1}{3\times2\times1\times5\times4\times3\times2\times1}=\frac{40,320}{720}=56$
Notice that, if you decide to choose five of the eight positions for girls, the result remains the same:
Combination of 8 distinct childern taken 5 at a time:
$_8C_5=\frac{8!}{5!(8-3)!}=\frac{8!}{5!\times3!}=56$
