I understand where you are coming from, and in truth, while a professional carer could never get away with this, and it is both unethical and potentially dangerous, I wouldn't be surprised if people do this and get away with it. However, I think Christin has hit the nail on the head - it almost certainly wouldn't have the desired effect (to keep the rest of the family from being disturbed).
You should definitely be able to get a disabling device for the gas stove, although how expensive that will be will depend on your stove. When she tries to make a cup of tea, does she use a kettle? It may be necessary to remove that from sight as well. As to the washing of the bed pad - I don't have any idea about how to stop that. People with dementia will do this sort of thing, believing they are helping.
I would be concerned, though, if she is wearing pads, how the bed pad itself is getting soaked through. You say a carer comes in twice a day, but I assume the pad she is wearing is being changed more frequently than that? It may be that you need to investigate another form of continence pad if it is being soaked through regularly. There are many different kinds and it may be that if her bed pad isn't getting soaked she won't feel the need to wash it.
Also, you say this has just started happening. How recently? Is it possible that she has something like a urinary tract infection (UTI)? That can cause a massive disruption in behaviour. Have you spoken to her GP about the wandering?
I realise that this probably isn't what you want to hear. It may be necessary to "granny proof" your home in the same way as you would "child proof". The problem is, an adults height and reach makes this much tougher. Wandering tends to be a stage although it can be a very looooong stage. Would if be possible to stop her accessing your and the childrens's rooms rather than the other way around?
Have you tried any visual clues? For example, putting a sign on the inside of her bedroom door along the lines of a shop Open/Closed sign, indicating that she should or should not come out of her room. Alternatively, and this may still trigger disruptive behaviour, some of the american sites that sell products for people with dementia, sell door size posters that visually disguise a door (e.g. a picture of a book case) so the person doesn't "see" it as a door.
Take care
