If they really are bengals (meaning they have papers, not just vaccination papers but those regarding their geneology etc.) then congratulations, they are bengals. ;)
But even with 'normal' cats one should always be aware that they should and will use all 3 dimensions of their territory... especially so if they are indoor only. Its up to us humans to give them as many options to go up and around as possible.
To your individual points:
Claw at the carpet on the stairs, slowly but surely ruining the
You didn't write anything about the available scratching options, so I'll cover the basics. Cats scratch both to mark their territory and slightly use up their claws. Leaving marks feels really, really good to them. Therefore they need suitable scratching options in key places. Key places being: next to exits/entrances, next to well-visited places, along commonly used walking paths.
Scratching options include verital and horizontal ones, such as:
For the carpet on the stairs, you can offer something attractive next to it, remove it or replace it with something sturdier (f.e. sisal again) that they are allowed to scratch on.
Jumping on fabric dining table chars leaving 'puncture' holes from
Impossible not to do when you are a cat. They will use every surface they can and since they walk on their toes, naturally their claws will leave puncture marks on materials like f.e. soft leather. This is normal, and can not be changed via training or other means.
Compare it to complaining that children leave fingerprints on things they have to touch, f.e. doors.
Constantly going after any food they can see or smell and that they
A common issue in many households, with a very easy fix. If they are constantly begging and grabbing anything they can, give them more good-quality wet food - as much as they want. They might puke from it at first because they are (and please don't take this as an insult) not used to being actually full, but cats naturally have a good sense of how much they should eat... These two just might have to rediscover it.
Good quality wet food is not Hills or something like that. Always look at the ingredient list. If they write down Exactly what from which animal is in there (f.e. Chicken (30% meat, 15% necks etc. - not an actual wet food example) then that is usually awesome quality. Medium and very decent are lists where there is no percentage, but still names all the different parts that are used. The 'bad ones' are the cans that just say 'At least X% chicken' and then mostly grain... You have no idea whats in there, the manufacturer makes no promises of whats in there and cats don't digest it well and get hungry again very quickly.
Don't feed dry food.
Human food should of course never be left unsupervised.
Somehow managing to open cupboard doors and boxes and pouches to get
at their food (apparently, when I was told about this it sounded
rather impressive and unbelievable)
Again pretty common actually, but easily fixable by childproofing the cupboard doors, using better boxes and not leaving pouches with food around. :)
Fighting with each other
Define fighting. Is one of the pair growling, screaming and hiding for a period of time afterwards? Or does one of them hiss/growl/meep only a little and the other stops, then the first one jumps in again? Or are they just wrestling?
The latter two are perfectly normal play behavior, only the first one is problematic.
Interacting with the toys of their new 5 month old son (sniffing, licking, eating etc)
Again, cats will use everything in their territory as they wish. Once more you can compare this to a toddler playing with scissors that are left on the floor. Of course it will. The question is: are the toys dangerous for the cats, meaning are they safe to be licked (I would assume so since they are childrens' toys?) and can they be accidentally swallowed. If yes to the second point, do not leave the toys out unsupervised.
Jumping up on the sofa and places where they are not allowed
Outside, cats have territorries that span many kilometers... with us, they only have a fraction of that. It is up to the owners of a cat to make that territory as exciting and usable as possible.
Why would the sofa be off limits? There is no reason to. If its fragile or easily punctured by walking over it, put a blanket on top, there are some quite pretty ones out there to protect sofas for just this (and other) reason.
Other places, same thing. Cats live 3dimensionally, you can at most teach them 1-2 places they shouldn't be when you are there or currently using it (for me, thats the stove and in front of the computer monitors f.e.). This is done by putting them down each and every single time when they go there... And by offering good/suitable/better/comfortable alternatives next to the place. For the stove/kitchen work area, I have bought a cat tree so they can watch what I'm doing (which is, after all, usually the purpose of jumping up there) from a safe place.
Frequently ruining "nice things", the latest of which was a very expensive bed sheet.
By now these sound like very bored cats that play with what they have available... How often do people actively engage them? Do you utilise clicker training? Bengals especially are incredibly active and intelligent, they need to use both muscle-types to actually be satisfied.
Some people use large wheels for their cats too, but thats 'just' running and gets bland/old. Get new stuff, change things up every now and then. Large feathers/small bells/other materials and things to run and chase after with their humans, labyrinths with little treats inside, agility training etc.
They jump up on me, such as my trousers and shoulders when even though it hurts, I'm not that bothered about but my mother hates.
So, like dogs do? This one can actually be fixed by training. Immediately go 'owowowow', no in a loud (!) but kind of high pitched tone and turn away if they stand on their hind legs and put their front paws on your trousers (if it hurts). Ignore them for a few seconds. They will get that this hurts and you don't like it.
If they do this out of excitement, use clicker training to tell them what to do instead of what not to do (f.e. sit) and then praise like they just won an olympic gold medal :)
Sorry for the loads of guesswork in my answer, I can only work with the details you provided.