When Is the Right Age or Weight to Switch Car Seats?

Looks like you are getting the same advice I would give, but let me tell you, I understand the mother in law situation! We actually had to tell a relative they were no longer allowed to take our 5 yr old places, as they decided her carseat was not necessary. Luckily, she was smart enough to tell us that "so and so" forgot her seat.
Please remember to do what is in the best interest of the child. Older generations just do not understand the carseat thing, but they are vital! Cars are made differently now, and the seats are designed to reflect that. There is a reason children should be in those seats until 4 foot 9. It's for their safety!

For your 9 month old: 1 year AND (not or) 21 lbs. They must be both! Dont rush this...Rear facing is safer for the baby. My daughter was 26 lbs AND 1 year before puting her forward facing. She outgrew the carier so we bought a seat that would transition and put it REAR FACING until she turned 1 year.

As for the 4 year old. They are required to be 4'9"...use a booster that has the head and neck suport not just the seat they sit on they are not safe in an accident.

Yes I know it is a pain...but it is better to be safe than sorry! I would rather be inconvienced than have a baby with a broken neck or worse.

I would leave all children rear facing for as long as the particular seat you use supports rear facing. It is the safest way for them to ride. Too many parents rush to turn kids forward-facing at 1 year and 20 pounds, and it just isn't in the best interests of most children. That said, I have a child who gets carsick...facing backwards exacerbated this problem while facing forward allows her to have a much calmer tummy and a more pleasant ride. But she was well over 1 and above 20 pounds when we turned her around. Even so, I didn't intend to turn her forward facing until well after her second birthday, but she needed to turn earlier.

http://www.car-safety.org/rearface.html

After that, I'd keep kids in the back seat forward facing in a 5-point harness for as long as possible. I wouldn't move them out of a booster until they're over 4'9".

Also, for rear-facing, make sure that the harness straps are at or just below the child's shoulders. For forward-facing, the harness straps should be at or just above the shoulder. This is important due to the physics of a crash.

For the rear facing seat, I switched my little one at about 1 yr-her legs got so long it was really just time to switch. As for your oldest my girlfriend told me 8 yrs or 80 lbs whichever comes first but no matter what their legs have to bend at the edge of the seat of the car, if they don't they should still be in at least a booster. As for no car seat at the age of 4/5, your mother in law needs to get over her disgust with car seats as it is your child's life she is endangering and that is not an exaggeration by any means. We all hate carseats-I can't wait until my little one can buckle her own just to save me the hassle but we grin and bear it, your inlaw should too or no outings with the grandkids.

A SUGGESTION
My mother in law bought her own seat for her car that has a buckle that she can easily do and undo(she used ours once when babysitting and spent an hour in the walmart parking lot trying to figur out how to fasten it)

Everyone should have to watch these videos....

This will make you reconsider forward facing your child before they absolutley have to......

http://www.youtube.com/watch?v=kRP7ynNI8mI&NR=1

Please consider keeping your toddler in a 5 point harness until 80 lbs with a Britax Regent (formerly callled a husky)Hopefully more carseats will be making them soon...

http://www.youtube.com/watch?v=azgBhZfcqaQ&NR

http://www.youtube.com/watch?v=y2LFo8vVi04&NR=1

To move to forward facing seats they must be both 20 lbs AND 1 year old. My son was 21 lbs at his 9 month check up and will just now be facing forward this week, when he turns one.

They suggest kids being in booster seats until they are 4'9'' but I don't think they have a weight limit they must also meet. I am unsure of what the law exactly says here though. I know a couple of 8 year olds who are shorter than that and don't sit in boosters, so I don't know if they allow for parental discression here, or not.

The seatbelt needs to come just below his shoulders for your son to not need a booster, but make sure it is comfortable and not rubbing on his neck. It needs to support his chest in an accident, so make sure it isn't too high. I also switched my baby boy at ten months and he was 20 pounds. He was almost too big for his old seat when we switched him, but by doctor standards, he was a little small. I didn't worry too much, because it has more to do with neck support than weight, so if you think he is ready, trust your judgement. If you don't think your 4 yr old is tall enough though, I would wait and make sure your mom in law buckles him in the booster seat, too, because you can never be too safe.

I'd get a convertible toddler seat for the 9 month old and keep him rear-facing at least until his first birthday. For the 4 yr old, he still needs a booster, but should be able to use the backless booster.
As far as the mother-in-law, she shouldn't be allowed to take the child anywhere unless she uses the carseat. End of discussion. You can tell her that if something went wrong while the children were in her care, she'd forever feel guilty for not following your instruction (and the safety recommendations!) and you are trying to protect her from that possibility.