How to travel with DC(5) in car at night?

[Nedmund]

DC is in a booster seat with impact protection but her head flops forward when she falls asleep.

Does anyone know of any safe products that support their head and neck?

Do you mean a High Backed Booster? Some of them recline slightly which might help? DS has one which is isofixed into the car, he is secure with a seatbelt and then the whole seat can be tilted to recline, only a few degrees but enough to help. Does the headrest adjust a bit too to properly support the head?

A travel pillow?

It's a Joie Traver I believe and it doesn't recline. I've had a few instances of her slumping forwards in it in the past despite me pulling the belt tight but assumed she was too small and her moving forwards slowly so moved her back into an isize for a few months. She doesn't generally nap in the car so I hope it's no longer the case.

What seat do you have so I can take a look?

We had one of those travel neck pillows for DD, she slumped even in a high back booster and she slept a lot in the car.

I still think it is not that comfortble, one reason we didn't drive late for longer stretches but she always said she is fine.

How big is she (height/weight)? You might find that a different car seat is more supportive, either one with a harness or just a differently shaped booster seat.