Joie - Juva

Looking at their policy, they want explicit airline approved seats. That's an issue. At 16 months it will be 100x easier if you can find an infant carrier type seat that she fits into which already has the TUV approval. (Maybe Joie Juva?) There are barely any seats for the next stage which have it and when they do, they are usually old discontinued models and/or you have to order the sticker separately and it's a pain. It doesn't sell seats so companies don't bother with it any more. OTOH, quite a lot of infant carriers for whatever reason have this. And if they have the mark then I reckon you should be OK with the 40x40cm requirement. Or, if you have a US seat which meets the US airline regulation and has a mark specifying this, this might be OK to use. I would definitely get written confirmation of this if you can to save any arguments on the day of the flight. The other option is to take a CARES harness but this will probably not be supportive enough for such a young baby.

You could get a folding high back booster for the 3 year old (not ideal but as it's only for a transfer your risk appetite might be ok with it) like the Be Cool Foldy or the MaxiCosi Nomad. And then depending on the weight / height of the 18m old do they still fit in an infant carrier? If so you could buy a cheap one like the Joie Juva. Or Joie Steadi to travel with.

At that age I'd pick up a Joie Juva for £60 and just take it for the trip - it is fitted with the seatbelt so you won't have to worry about the vehicle having Isofix. Doonas aren't a good stroller option as it promotes baby being in that curled up car seat position for longer than necessary.

Idk girl but I’m a FTM and its on my registry. Looked at joie then looked at nuna and we said “fuck it, we ball!”

I tried them both out in person! We did Joie first and it was fine absolutely would have worked. Then went to see the nuna and we could tell the difference in quality. My husband lovedddddd the Nuna Swiv and it was so lightweight and easy to move around compared to the others it made the choice really easy.