Let them see you taking care of your teeth, and make sure that they do the same thing. If they have a snack, have them brush afterwards, even if it is just with a wet brush to save money on toothpaste. There are mixed opinions about fluoride, so I'm not going to get into that. If your children's teeth have crevices that are prone to cavities, it is worth the money to have a sealant put in them. Dental work is extremely expensive.
Prevention is cheap.
Don't waste your money by neglecting your teeth.