I do believe the Admin asked you to stop with that last statement if you couldn't prove it. The FAA recommends car seats for children. They don't require it because they fear more people would drive rather than fly. In this case, OP *is* going to fly there- therefore the car seats are recommended.
And our travel vests pass out standards here- you have no business saying they don't provide any safety. They act as a booster by keeping the seat belt in place- which is exactly what you are promoting- booster use to keep the seat belt in place.
A source was provided which can easily be found from the reference. What more do you want? It doesn't matter what people or organisations recommend if it's not correct. The source I provided come from FAA themselves.
If every single child on an US airline would have used a car seat in the past 20 years(!) not a single life would have been saved. I say 20 years since this the data was a few years ago. From FAA:
FAA’s position is based on two fundamental premises plus the empirical evidence, which shows no preventable infant deaths in air carrier operations for more than 17 years.
Perhaps travel vests pass the "standards", whatever these are (please provide a link to the standards), but this says nothing. A safety vest provide no safety advantage and does not avoid a child moving around. Makes a parent feel good but does not protect a child better. Since the travel vest offer such a great safety advantage then please provide some
peer reviewed research or statistics which show the vests are superior. Not thoughts, opinions or feelings about their superior safety. Since the vests are so much better we should be able to see hundreds of lives saved from the references provided.
The kids are 4, 4 and 3. Twins will turn 5 while we're there.
That is helpful info. The easiest solution would be to make a call to the rental place and ask if airbag in front seat can be turned off. Also ask if middle rear is a lap belt. Quite crucial info. If airbag can be turned off it's a great and safe solution which will save you money. If middle rear is a lap belt then we have a problem.
Most people also don't seem familiar with a Ford Fiesta. It's a SMALL car and definitely not made for for five people. Especially not for three kids with car sets in the rear.
If we look at compromises we see from research that keeping your older FF kids in seat belts, only seat belts, is around 70% less safe than using a hbb or lbb. Not a god solution, especially not in the horrible traffic in Brazil, but we also sometimes have to be humble and accept some compromises. Looking at the positive side it's a lot better than keeping a three year old FF which is 500% less safe than RF.
Are you by the way sure you will be getting a Fiesta? I have traveled to 60+ countries and rented a car in quite a few of these. Options are always "Fiesta or similar size/class". Apologies if you are doing a long term lease since car model might then be easier to specify.
