I think that when it comes to determining what was used for penetration, size would be the main issue. So yes, a douchetip would be close to - and as smooth as - a finger in size and contour. Shape (whether of a finger or object) would come into play if there were definite sharp edges, ridges, etc. In a child's vagina, a douchetip would probably be sufficient to cause chronic inflammation and push the hymen aside - even though a douchetip is smooth. Even so, rough douching would have abraded the sides of the vagina and contributed to inflammation.
Digital penetration, in and of itself, doesn't point to either an adult or child: a small penis, a large finger, a woman's finger...impossible to tell which it was (although a woman's finger might have had longer nails than a man's, producing a recognizable pattern of abrasion). Penile penetration by an adult male would typically result in a greater degree of inflammation and enlargement of the vagina, but so would penetration by an object - or more than one finger. It could be that a combination of all of these was involved in JBR's situation.
I am not a doctor, of course, but I have some acquaintance with these matters because I transcribed autopsies/pathology/sex abuse reports for several years at local hospitals. Technological advances might have made it possible to be more specific nowadays, but at the time of JBR's death, this was probably as much as could be deduced.
