Definitely, no. Aspartame or saccharin does not contribute to tooth decay.
Bacteria. Bacteria causes tooth decay by converting sugars into acids that etch the calcium out of your tooth enamel. If you're asking should you give children artificial sweeteners, check with your pediatrician. To prevent cavities see dentist regularly, brush 2x/day or more 2 full minutes each time, have a balance Fluoride regimen, clean between teeth daily, and consider sealants. Ask your dentist.
No they do not. Tooth decay is the process in which the minerals from which the tooth is formed are dissolved away in acid solutions, usually in bacterial plaques from not brushing and flossing effectively, but also from any other acid food, drink or substance placed into the mouth. Now i also think it is important to make the distinction that sugar does not cause tooth decay either, but failing to remove it.