Booster seats for kids?

[lime1]

Having sussed out the issues with boot space I now need to plan for the back seats for my upcoming trip to the US. I have 2 boys aged 6 and 8. Will I need booster seats for them or will the back seats be okay without? They have backpacks which double as booster seats so I could take these if needed but easier to travel lighter if they're not required. Thanks