How can I be sure that my LO flosses and brushes all of her teeth?

According to your profile you say that your wife has dementia, so it's unrealistic that you expect her to brush and floss her teeth on a regular basis, as her brain is broken and I'm sure that she's forgotten not only how to, but the importance of it.
If you're wanting it done, you will now have to start doing it yourself or your son who looks after her as well.
And of course as with anyone who has dementia, it's important to choose your battles wisely, as not everything is worth fighting over, and this may just be one of those things.
When my late husband was completely bedridden with vascular dementia, I was lucky to get him to brush his teeth once a day, and I had to just be ok with that, as to me it just wasn't worth the fight anymore, as I knew in the big picture of things, whether he brushed his teeth once a day or 3 times a day, it wasn't going to change the final outcome of his disease.
So just keep that in mind when you're choosing your battles.

Not using a Water Pik properly? Really?

My mother never flossed her teeth a day in her life and died at 92 with one cavity.

Pick your battles, because if this is the hill you choose to die on, you'll be gone long before the war begins.

Every day your LO is alive that she's not fussing, screaming, baring her teeth at you cussing, or ripping her incontinence brief off and making a huge mess with the contents is one day to be grateful for small favors. Let go of flossing and water piks and all the things you'd expect from a person without dementia and who cared about her teeth in the first place.

Consider yourself lucky if she can brush her teeth at all, or if you don't get a huge fit thrown when you try to help her with the toothbrush.

Pick your battles and let ALL the rest of everything go.

Also realize there is such a thing as "overkill". Recent studies show that regular brushing AFTER MEALS is actually dangerous as the enamel in the teeth is more vulnerable and just getting brushed away. There are some excellent dental rinses your dentist may recommend to prevent gum disease, the largest cause of tooth loss in elders. Periodontax would be an example. Speak with your dentist if this is of a concern.

In stead of water in the water pic, ask your dentist about a rinse. The dentist will be able to recommend something good. At least she can get that in her mouth, and believe me, if that’s your biggest battle, you’re doing great!

Like others have said, with dementia, you have to pick your battles. Eating right, taking meds, proper toileting, proper hygiene, a bit of brain stimulation exercises, as well as some physical exercise and last but not least, love and companionship are your priorities.

As dementia progresses, those get more and more difficult. You begin by “letting up” on let’s say, exercise, then hygiene starts to lax. Mind you, you don’t give up, just slack up. Brushing once a day is fabulous. When she starts to balk at the waterpic, then waterpic every other day or three times a wk. If possible, offer to do it for her, but be prepared for a battle. Battling cause every one stress, so, in time, you give up a little more.

Thats dementia…you give up a little bit at a time. Both patient and caregiver. There really isn’t a lot more you can do. Battling isn’t worth it. You can try to make things more “enjoyable” for her, have her sit in a chair, put on some music or TV, then waterpic or brush her teeth for her. You will need to try different things to see what works. However, be prepared, because eventually, nothing will work.

Then it’s just about making her comfortable and loved and safe. That’s the only part that you don’t slack up on and even that can be difficult, if the dementia person is hostile. But you do the best you can. Good luck. And if you find anything creative things that work, post it here to give others ideas.

Maybe now you will have to help her.

I had an aide tell me Mom was not brushing her teeth. I asked the aide if she was putting the toothpaste on the toothbrush and handing it to her. I could tell by the look on the aides face she wasn't. I had been doing this for almost two years before Mom entered the AL. I thought it should be logical, if they can't do it anymore, you have to do it for them.

This is Dementia. They forget how to do the simplest things.

P.S. I don't brush my teeth for 2 min. Brushing for 2 min does not mean she is doing it right. As long as she is brushing every surface she does not need to brush that long. Actually, too much brushing and too hard can cause the gums to recede. I think a good brushing before bed is good enough. It is important to keep up on it but no need to be so strict.

How she does her teeth will not stop her dementia, I would suggest that you stop trying to control her let her be, she is doing the best she can under the circumstances.

Let her be, don't put her under any undo pressure.

Have her try the Colgate Wisp disposable toothbrushes. They are mini pre-pasted disposable toothbrushes that require no rinsing and can be used anywhere and anytime. She can take her time without anyone standing over her. These small brushes also include a pick on one end.  https://smile.amazon.com/gp/product/B071DPCBQG/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

lepleydm: Seek dental professional advice.

Offered to help her. watch and point out the areas that she’s missing - let her do it. Be loving and supportive about it. If that doesn’t work perhaps mention there’s food in her teeth even if there isn’t just so that you can check her level of concern. Eventually you might have to do it for her, in the long run.