It's nothing to do with trans or not trans it's the simple idea of putting opposite sex people together.
Opposite sex perhaps, but same gender. It makes a difference.
If girls normally have a woman sleeping with them and it's now going to be a person who was a man but is now identifying as a woman then yes of course the risk of something bad occurring must be slightly more likely.
Why is it "of course"? As far as I can see that's not a logical conclusion to make at all. Why would someone who has a high chance of being severely uncomfortable with their genitals be more likely to put them to some nefarious use than someone else? If they're on hormones then their genitals may not even ahem... operate... as usual. And it's quite probable that a trans woman will be very aware that some people hold beliefs that they are some sort of sexual deviant, and thus be hyper aware of anything which could be misconstrued as inappropriate.
The only way I can see that you could see them to be more of a risk would be if you believe that predators are pretending to be transgender in order to gain access to children. The odds of this actually being the case is vanishingly small. Prejudice against trans people, especially women, is still painfully common. I find it very hard to believe that anyone would go to the lengths of changing their name, documents, coming out to family/friends, changing how they present themselves, and maybe even taking hormones, just in order to get access to children on the off-chance they could abuse their position without getting caught.
If you're saying that a trans person has biologically changed sex then I disagree.
Their chromosomal make up may not change, but in almost all other ways they will. Hormones and surgeries are powerful things.