I believe anythig can be harmfull if not used correctly
Like sevindust. Only use that on the floors around the racks and cages. NOT on the snakes theirselves or in the enclosures. Thats way to risky. I understand people may have used it on the snakes but I personally would not. That is just my personal opinion on using a strong chemical compound on an animal I am using to produce healthy babies.
As for losing babies to provent a mite. The directions clearly say to NOT put it directly on the snake and to NOT use in any enclosure that does not have bedding or substrate. (this does not mean newspaper)
You also need to let the tub or enclosure air out and completely dry (for at leat 15 minutes IMO) before putting the snake back in that enclosure and be sure the snake has proper ventilation even after that. Proventamite is only for treating the enclosure and tub slots. Not the animal itself.
No I wouldnt use it on a hatchling of any kind but if you follow the directions and do not cut corners at all, you should have no problems.