landing on you is probably a positive, instead of the ground.
This has been done before, the only thing with being lower is that you might have to deal with some splash from rain on the ground, or be a bit low and cop* some wind blown rain.
ooh- I just re-read and saw that your son has a bridge hammock. Be careful about him being higher in a steeper set up and the edges of his poles poking the tarp.
*is this an Australianism? what I mean is that you get hit.