I have 3 children, 16 mos old, 3.5 years old and 6 years old. I want them to all learn French, but since they are different ages I use different approaches with the same activities. For my 16 month old I think just listening to us all speak and practice French is great for him. For my 3.5 year old daughter and 6 year old son I use about the same approach of doing activities together in French that I've described in other posts, but with my son I give him challenge exercises too.
Let's say I were to teach them colors one day or reinforce their colors. With both I would do about 8 basic colors, but with my 6 year old I would add about 5 more more obscure colors. So his vocabulary will expand more to his age level. My daughter is always there when I teach him the extra vocabulary, and 9 times out of 10 she usually picks it up without me actually teaching her, but this way she won't get overwhelmed with too much French. If she's open and able to learn them, she will.
One of the most important things I've learned about teaching my children French is that you should never push them so much that they stress out about it. You never want it to be a negative experience.