My little one (currently aged 9) only granted to sit in the front.
My girls only only stop using the car seat when they aged 7. And little babies should be using baby basket.
I saw too many drivers not using car seat, or sitting the kids in front....or without buckle...too dangerous.
My friends are all very cautious on this as well....they are doctors and experienced so many car accidents at the hospital...they won't allow their kids to sit in cars without air bags...and not to mentioned....sit in the front when they aged below 10.
HK don't have any law to compel children to use booster seat or baby seat. Children are free to take the back or front seats and driver will held no legal responsibility.
Nonetheless, in US, it requires children to have booster seats until reach the age 8 - 12 (depends on which states). Some states required children under age 12 (and certain height) are forbidden to take front seats. It is all down to safety.
At present, my kid is 7 and I still need him to take booster seat at the back. I know what a great, saftey awareness driver we are. But you will never know the dirver of other vehicles.
Before answering that, my kid used to take back seat and wife at the front, i.e., nobody sitting next to the kid. So, I would take option 1, 細仔同大仔坐中間行, mami坐前座.
Having said that, depends on your younger and elder one, are they used to have an adult seat next to him/her? If positive, it will take some time to pursuade your kids that no adults will seat next to them.