Re: Booster training
I started once DD was mature enough to sit correctly for the length of our common trips.
I planned to start in June before Kindergarten, because once in Kinder there was a good chance that she would start riding in other people's cars from time to time and boosters are much more user friendly than carseats (I can trust other people to put a common booster in correctly, but not a harnessed seat).
The best laid plans of mice and men . . . DD broke her arm in March, so we started then. She was 4y almost 6mths and only 34lbs. She rode perfectly for all rides up to about 1.5hrs. She now rides in a booster in all secondary cars (daycare, gma, DH's Miata, and our secondary car). She is 5y1m, 41" and 36lbs now. She can sit correctly for rides up to about 4hrs.
However, I am not of the "harness forever" mindset. I harness until booster maturity. The only reason DD is still in a carseat in the main car is because we take multi-day roadtrips and I don't feel that she is mature enough to stay in position 100% of the time during those trips. That seat is rfing by her choice.
S, mama to my Second Grader, K (9/07) and my Kindergartener, e (7/09) and married to my best friend, E (2007)