The number of arrangements of the seats is (# of possible choices for seat 1)(# of possible choices for seat 2)...(# of possible choices for seat 6).
There are 3 choices for the first seat since there are 3 boys. With that seat covered, there are 2 choices for the third seat. Likewise, there are is only one choice for the fifth seat.
Similarly, since there are 3 girls, we have 3 choices for the second seat, 2 choices for the fourth seat, and 1 choice for the sixth seat.
So the number of seat arrangements is 3*3*2*2*1*1 = 9*4 = 36