(1/2)
As you can probably summarize from the responses so far, 'models' seems to convey more respect and professionalism than 'girls'. Of course, like all the opinions above, mine is simply a personal view on the matter. Speaking as a linguist, I can state fairly confidently that language is what we make it be - but also what it has been in the past. For instance, when I think of 'a girl', I associate the concept with: 1) children 2) women who are not afforded respect as fellow adults. I base this knowledge on how I have seen the word being used in my personal past, within the social situations that I have observed from my point of view. This is not universally true, though. Some responses here do not see any problem with calling a grown woman a girl and may not even associate the word with a lesser status or any sort of unequal power relationship. Thus, there is no absolute true answer to this question: both 'model' and 'girl' mean different things to different people and the best you can do is to gather as much data as possible and make your call based on the knowledge you have.
I understand that there is some resistance to changing language 'artificially'. Calling a woman a model does not, indeed, change how you think of that woman now if you have thought about her as a girl for years or more. Change is, in most cases, more gradual and elusive. If you indeed need to refer to performers of various sexes and genders with one term, though, 'models' would likely convey what you need with very few problems, thus affording equal status to your performers.
Oh, and there is one thing in 91hebasu's post that I agree with: you could actually ask your performers. My personal hypothesis is that some of them might find girl more flirty, but I am sure you would get as many answers as you have models. English may not be their first language, but that does not mean that they do not have valuable opinions. The same goes for you, obviously.
TL;DR bonus, about language and neutrality:
There are a few posts here, like the post by The3LeggedMan, which do not see any real difference between the two words. I agree in the sense that if you call your performers girls, very few people would truly associate them with small children first. That being said, I do think that there is a subtle difference, related to language and power (I recommend reading critical discourse analysis theory if you want to study this further). Fortunately, it is possible to inspect this difference via linguistic intervention: we can substitute one word with another and see what effect it has on the meaning of a sentence. Consider substituting 'a man' with 'a boy' when talking about a grown adult. Let's look at a few examples. Why would you not let Matthew handle your finances in the sentences below:
"Matthew is a sweet man, but I would not trust him to handle my finances."
"Matthew is a sweet boy, but I would not trust him to handle my finances."
Similarly:
"Selma is a sweet woman, but I would not trust her to handle my finances."
"Selma is a sweet girl, but I would not trust her to handle my finances."
For respect:
"Anna is my secretary. She's a talented girl."
"Anna is my secretary. She's a talented woman."
"Matthew is my secretary. He's a talented boy."
"Matthew is my secretary. He's a talented man."