If they were consistently low in Vitamin B12 it would affect their overall growth. I need B12 injections, and one of my breastfed children developed a deficiency as a baby that wasn't picked up until she became failure to thrive. She'd stopped gaining weight and was lethargic at 7 months old, but another significant symptom was that she stopped growing taller and there was a hollow in the back of her head, where those three skull bones are supposed to fuse. Famine-starved children do not stop growing taller, even when they lose weight, but a B12-starved child will almost immediately experience arrested growth. This makes me sound terribly negligent, but we just didn't know, and it was so easily fixed at 7 months old because we were WATCHING our child and consulting doctors the moment we thought something was wrong... Other symptoms of B12 deficiency (that a baby can't tell you) is that your mouth and throat are always sore, so my baby didn't want anything except familiar breast milk. Long term B12 deficiency can also affect emotional responses and the nervous system.
So I think they didn't eat much meat/ animal products, and didn't have the basic nutrition to grow and develop normally. The cut and style of those dresses would also seem designed to bind down/ camouflage even a little sign of breast development - they are definitely "little girl" dresses, rather than dresses worn by a small-breasted woman.
Other observations: I have a new respect for Elvis impersonators who take on clients like that and run with it. Those guys in the videos were all true professionals, helping the kids participate and never losing character for a minute with even a flicker of "Wow, these people are weird." Even in the world of Elvis Vegas weddings, this one would be out of the ordinary. The wedding video reminded me of a kid's party where you hire an entertainer who just takes control and everything goes seamlessly. Only the "kid" in question was the mother.
I noticed that the kids had trouble coordinating and singing (I choose to blame a B12 deficiency) but it also seemed to me that they didn't know the words to the songs. This seemed weird and sad to me too, because if the mother was so fixated on Elvis, surely she'd be playing the music regularly at home or in the car so the kids would know the words. There's another sad aspect to their childhood.
I am recalling a case in Australia where the father had a decent job and was well-respected yet behind the scenes the SAHM simply couldn't cope with 2 children and opted out. One child died when he cut his foot in the rubbish-infested house. While it seems a very different case, I think Louise was completely out of her depth and couldn't admit it, she couldn't deal with adult children so she consciously and unconsciously condemned them to an eternal childhood. She had a fantasy to fulfill and she was fixated on that. Now that sexual abuse seems to have been ruled out, my gut feeling is that she was the driving force behind this, and David didn't risk tarnishing his shining armour by taking the children's side. Although I think they both must be emotionally stunted to keep having babies to reinforce their own beautiful love story when they couldn't seem to interact normally with any of these kids. As someone else mentioned, there is no interaction with the children. Elvis came across as more of a loving parent than the two of them put together.
Again, it's Elvis who has finally got me crying in this. I just want to thank him for giving those little girls a moment of feeling special when he lined them up and told them to each step out into the chapel one at a time - those girls were beaming and they stopped for an individual photo in front of the camera and then kept going, just to blend into the background of their parents' fourth/ fifth special day. Elvis is probably the only one who ever treated any of those kids like an individual.