Traveling with toddlers

Hello! I will soon be taking a 6+ hour car ride with my 2 month, 20 month, and 3 year old. We are hoping to drive through the night to minimize stopping.

I am looking to you lovely people of beyondthebump! I want to get something I can put in my kids car seats to keep their heads from flopping forward. I was thinking neck pillow but I've read that they push the head too far forward.

They are both forward facing and in diono radian car seats if that helps at all. Anyone have any experience for what works or what doesn't?

Oh, I should add I understand car seat add ons are not safe. I am up to anything short of strapping their heads in.

Thanks a bunch!