Are you asking if your child is too young to play DS or if the DS is aimed at younger children? The way I'm reading your question, you are asking if the game system is for younger children, but everyone else is reading it the other way!
To answer your question- your children are the right ages. There are games for all age groups. My kids are 5 and 9 and have had NDS for about 1 1/2 years. We bought them the older style, used at the game store. They each have one and are able to link them together for some games and can also "chat" on them with each other. My 5 year old can't read, so they draw pic's for each other. There are also games for older people. I love the brain age games, and I typically HATE video games.