You don’t actually need either permutations or combinations for this one. Notice that if you know where the girls are sitting, you know the whole arrangement, since every chair is filled. Thus, to count the number of possible arrangements, you need only count the number of different ways the girls could sit. Since there could be any number of girls from $0$ through $8$, the girls could sit in any subset of the $8$ chairs, from $\varnothing$, the empty subset $-$ no girls at all $-$ to the entire set of $8$ chairs $-$ no boys at all. A set with $n$ elements has $2^n$ subsets, so there are $2^8 = 256$ different sets of chairs that the girls could occupy, and therefore $256$ different arrangements of the $8$ girls and boys.
(If by any chance you’re required to have at least one of each sex, you have to rule out two of these arrangements, the one consisting of $8$ boys and the one consisting of $8$ girls; that of course leaves $254$ arrangements.)