I Am A Grandmother Preparing To Take Two Toddler Grand-daughters

I am a grandmother preparing to take two toddler grand-daughters on a fairly lengthy trip. Do I need to worry about car seat asphyxiation since their heads flop on their chests? Their car seats are up-to-date and they are securely strapped in. They are 2 and 1/2 and 3 and 1/3. Thanks so much...it's my "watch."