I've got the same concern, except I'm lead to believe that my babes were kept indoors in their previous home, so I'm terrified of letting them out. Peanut has been in the garden a couple of times under supervision, but did escape under the fence and I nearly had a cardiac arrest!
She came back after about 5-10 mins and I've not let her out since! Cooper on the other hand, is a big girl and he won't go out. He's stepped outside once for about a millisecond and then flew back in.
I'm so, so scared to let them out, so I 100% feel your distress. I would like them to enjoy the outdoors, would also like to get rid of the stinky litter tray!
They've been with me for 5 weeks now