I don't know how old your kitties are, but if they're between 5 and 10 years old and in good health, then what you may want to consider doing is - if you can afford it - having the vet do the teeth cleaning and then beginning the maintenance. It's just like with people - I brush twice a day and floss twice a day - but each year I still need to get my teeth cleaned by a pro. Cats get that build-up too.
We have one kitty prone to plaque problems - we get her teeth done every year since she was two years old. We brush her teeth (she loves it) and give her special treats - but she still lost two teeth at five year old.
The vet can do "pre-anesthetic" testing to make sure there won't be a negative reaction to the anesthesia.
As artsick pointed out, there are special treats to help remove plaque from cats' teeth.
Do you brush your cats? Do they like to have their cheeks brushed? This is how we started - by brushing their faces with a regular brush. When they came to absolutely love it, we then used the toothbrush on the outside of their mouths. Spooky, the one with the plaque problem LOVED it - and would chew on the brush. So we just started brushing her teeth and massaging her gums with the baby tooth brush, and she just purrs and purrs and purrs.