S.K.
Joanna's advice is right on target! By law- a child can not ride in a belt positioning booster until at least the age of 4. Height and weight are also a factor- but it is also age. This is because your child's spine needs the right amount of time to develop regardless of your child's size.
Take that little angel out of a booster and keep her safe in a 5-point harness where she belongs.
Your 10 year old can be responsible for buckling/unbuckling when she is with you. Otherwise you'll just have to walk back there and secure her yourself. You'll be glad you did if you ever had an accident.
The two younger children can go in the two captain's chairs in rear facing seats. That way you can have easier access to them.
SO many people make mistakes when it comes to the safety of their children when in the car. There are a lot of crazy drivers out there- no matter how safely you drive- you can't control the other drivers on the road.
Also- don't be the mom who leaves the handle on the infant seat up!! That prevents it from working properly in an accident. Please move it down once the seat is clicked in.
I hope this advice is helpful. We moms need to stick together and watch out for each other. Without advice from moms who have been there before- we could never know it all ourselves!
Good luck with the new little one!
-S.