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Based on what you've said, I think you're going to have to do something that's not terribly imaginative; after all, the client is a children's hospital and the audience will be the online presence and print media. So it's an editorial or commercial shot, not an edgy kind of thing. You'll want to be able to clearly identify each of the individuals in the frame. First thing is to consider the location; is it in front of the hospital? Or out on the road someplace? If it's at the hospital, you should try to make it identifiable in the frame. I'd probably want to orient myself in front of the bikes, at a low level (say, around top-of-wheel height) the bikes facing toward me, with two of the girls seated on them and the third, with the child, in between. The age and ability of the child should dictate the pose of the girl in the middle with the child. I'd probably light them gently, with one big umbrella centered over the camera. I'd probably go for a light touch here, with a tight ratio between ambient and fill. Again, if the hospital is in the frame, that consideration will drive your baseline ambient exposure. After that, depending on the location, if the child is up for it, I'd then take that umbrella, move it up higher, drop the ambient and boost the light to give some additional drama to the shot. So you'll have two very different looks with the same basic setup.
Hope this helps.