DD2 lies on her tummy to sleep on a flat surface, so in that way we didn't have the same challenge. We transitioned by putting her on a mattress on the floor first though, because like you, I was worried about her getting off our bed too easily. My DD1 though was able to get off the bed safely from a very young age once she was awake. She just wriggled backwards and down she went. A monitor is particularly useful then! DD2 is not quite ready to get herself off the bed without a little oversight, but she isn't far off it. Once she can do that I could safely sleep her across ways and then let her get down when she's awake.

Not sure if that helps. We've recently made the transition from amby hammock to her junior bed (with a side bar to stop her climbing off).