There are so many factors that go into why kids teeth get stained. From not brushing properly to tooth trauma, certain medications...even infections in a pregnant mother can cause problems with tooth color.
The first thing would be to have a dentist identify what the problem is... plaque or actual discoloration... then seek assistance from there. We have had our little ones visiting the dentist since they were 18 months old... the dentist is great and informative and will give you tips on keeping teeth healthy!
My personal opinion is if your child's teeth are appearing discolored before that first 18 month visit to the dentist then there might be extenuating factors involved besides plaque build-up