There are five slots:

X X X X X

Where one of the kids go into those slots.

There are 5! = 120 make an arrangement.

In (a) we want B G G G B there are 3! = 6 to seat the girls in the middle and 2! = 2 ways to seat the boys. Thus there are 6*2 = 12 to make this seating. The probability is 12/120 = 1/10

In (b) then A B C D E or E D C B A where A is younest and E is largest. There are only 2 ways the probability is 2/120 = 1/60