Seated Shoulder Height!
What's that? It's the new buzzword in the carseat safety business. Simply put, its the height of the kid from butt to shoulder
Why is it important? When your kid who is within the height and weight specs of the carseat starts complaining about being uncomfortable or having back aches after sitting, it's time to whip out the tape measure. Two kids can be the same height but have different torso lengths which will affect how comfortably they fit into their car seats.
I know that when I bought the girls' Britax Marathons almost 2 years ago, Seated Shoulder Height was not an official thing. Now it is and details can be found on the Britax website here.
I measured the girls and Spice has a Seated Shoulder Height of 14.5 inches while Sugar is around 16 inches.
What next? I plugged the girls' numbers in the Britax "Fit My Child" tool and theoretically they're still good for their Marathons. However the maximum Seated Shoulder Height for the Marathon is 16.75 inches so I think new booster car seats are on the table before this fall.
If you have kids currently in convertible car seats, it's worth it to do some research on the recommended seated shoulder height for that seat and see if you're due for a change or not.