6 or 60 lbs is the legal standard, but FYI it is not really the safest standard; 8 or 80 lbs would be closer, and even that is an estimate. The issue is how the seat belt fits. Presuming a belt with a shoulder harness (which is what a child should always be using unless there is no other option...and if no shoulder harness, no booster), the shoulder part should go across the shoulder, not the neck, and the lap part across the hips, not the waist. Until they are somewhere in the range of 4'-6" or more, most kids aren't tall enough to meet both those standards. They should be able to sit all the way back in the seat and have their knees bend at the edge of the seat; this will differ from car to car. If this isn't the case, they are likely to slouch down to make their legs more comfortable, and then the belt runs across waist and neck. My 9.5 y.o. ds, 77 lbs., is still generally in a booster in our cars because it helps him be more comfortable and the fit is best; his 6 y.o., 47 lb. brother will be in one for quite a while longer. We keep an extra booster in the car for friends! Also, if using a low back booster, make sure that the top of the headrest is at or above the top of the child's ears.
Now that I've finished the sermon
I'd say that you should definitely send one booster for your 6 y.o., and if your 9 y.o. is small in height, I'd send one for him as well. They are inexpensive, easy to use, and light, and you will be driving a lot of miles at high speed. Have a great trip, and show those lucky grandkids how much you love them and want them safe. I just wish all grandparents would care as much!
