Please Help me with what option is safest?

[Blii]

Hopefully this is right place. I cannot find anything on google to help me with this.
So, I have 3 children ages 6 months, 4 years and 7 years.
They have all been in the back row of the car up until now. And now the baby has a new car seat all 3 seats don’t fit!
So my question is which option is safest?
7 year old in her high back booster on the front passenger seat (seat all the way back).
Or
7 year old on the middle back seat in just a normal booster cushion, not high back. This is the only way I can get all three of them in the back. The two high back boosters do not fit now I have the new bulkier baby seat.
My car doesn’t have the option to turn the airbag off.
Getting a new car is also not an option unfortunately.

I would go for the second option of your eldest being in the middle back seat on a booster cushion.

Plus I'm so careful with child seat safety. My 9 and 11 year olds are still in high back boosters despite both being 135cm + tall.

7 year old in the middle. Assume it has a head rest?

Out of interest why is the baby in a new car seat? 6 months is usually too early

I'd keep baby in an infant seat until they outgrew it, usually around 15-18months. That way you can keep the 7yo in a high back boosters for longer.

I'd also look at buying 2 new hbb as you can get some really narrow ones. Booster cushions are a significant step down in safety.

Britax adventure and besafe izi flex are both narrow hbb.

Thank you for your replies

@99problemsandthecatis1 we had two britax adventure hbb in the back with a maxi Cosi pebble and they all fitted perfectly. These hbb are the narrowest on the market
@NeverGotMyPuppy The pebble was on loan and the person I was borrowing it from needs it back as their baby is due any day now. My baby is still in a rear facing seat, and he can rear face until he is 4 in this new seat.
I’m disappointed they can’t all fit in the back as that is the safest option however I’m not sure if my 7 year old is safest in the back with just a booster cushion and no side/head protection. Or the front seat with her high back booster.
It’s a tricky one.

@Blii there's an amazing Facebook group called "a - z of rear facing" where they are incredibly knowledgeable and the admins are all trained fitters.
If you tell them your car make model, car seats and post a pic of set up, they will be able to advise on safest way to get fit them in.

Following your update, I'd buy a Joie juva for the baby (about £30) and keep them in that as long as possible before moving them in to the seat you currently have. For the benefit of the 7yo.

Then the 7yo is safest in the back.