I'm starting to potty train my child at 20 months by not sleeping with a diaper on. It gets tiring to change / wash the bed sheets every night. I was thinking of using disposable one time use puppy training pads. Is this a feasible use case for something that is meant for animals? I means its just a super absorbent pad or is there another suggestion to create a quick environment to change the sheets when they wet themselves during sleep?
No. Don't. Maybe they will work and maybe they won't but those items are designed for animals, not children.
Animals don't wear clothing while your child should be wearing clothing at night time. Wet clothing while laying in a wet puppy trainer is not going to go great in the way of hygiene.
If you are tired changing sheets, having to air out mattresses or futons, maybe your child is not ready for sleeping without a diaper at night time. 20 months is extremely fast for attempting this and I'd almost say it's counter-productive.
I see you are in Tokyo, so if wearing something that feels like underwear and less like a diaper is your goal, go to AEON or any local department store and buy an appropriate size package of メリーズパンツタイプ (pants-type diapers). A pack will set you back ~1700円. Only let them wear it at night time and let your child assist with putting them on. They worked wonders for both of my children who have already gone through potty-training.
Do not use a puppy trainer pad for your child. It's the same level of waste as a diaper.
I don't see why it wouldn't help. It would produce a lot of waste though and I believe they are thin and tend to bunch up and sound "crinkly", so perhaps look into what washable fabric options there are. I think looking up "hospital pad" or something similar may be a useful search term.
I use a fabric pad (it was originally a "hospital pad" my parents had leftover from when my sister and I were kids) for my dog to give her a place where she is allowed to go without getting in trouble while we are at work - it is sturdy enough that it does not bunch up and it washes super easily. This I think would work better than the disposable "puppy pads". (The puppy pads are usually sold with a "frame" to keep them from bunching up). I think they would be too easy to move out of the way when tossing and turning while sleeping to offer much protection.
Another thing to consider though: the quilt/top sheet. A pad may protect the bottom sheet, but the top sheet can still be soiled. (I have no answer for this problem unfortunately.