
Cosco Kids
Scenera Next DLX Convertible Car Seat
Super light, cheap, great for air travel, but outgrown fast.

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Uh no way am I skipping the carseat. We have the lightweight Cosco one and even when traveling by myself with the I haul it, our luggage and my 3 year old. I usually just rent a car wherever we're going and deal with installing it.
We also do the lightweight Cosco seat with a padded carrying backpack that we check. Set it up so the harness height is situated in advance and make sure you know how to safely install it. Depending on your budget and child’s size (under 45 inches and 50 lbs so likely for a 4 year old but it’s expensive to not be able to use a lot) there’s also the WayB Pico which is super light and compact for travel. i would definitely do a stroller for a 4 year old at Disneyland but would rent there through BabyQuip or at the park. I’d also gently encourage you to learn how to install all of your car seats. Kids can accidentally unbuckle them climbing in and you need to understand the install. Most manufacturers usually have videos on YouTube you can look up!
I'm a parent of 4 and we fly a lot - I'd absolutely bring the car seat on the plane. It's safer, and it gives her a comfortable familiar space to rest. We own a lightweight cosco carseat just for the purpose of travel. Also, there is no limit for amount of breast milk allowed. I'd buy a soft sided cooler that fits the carry-on size requirements and put the milk in there with ice packs. You can also bring a diaper bag for free with most airlines (in addition to your personal bag).
Yep just buy a lightweight travel car seat and haul it along (we like Cosco). We had a car seat backpack at one point but it was honestly heavy and overkill Not using a car seat would be ROUGH
So easy to just sling over your shoulder
We did this years ago when traveling to the UK to see family too, and bought a cheaper/lightweight second car seat to make it easier. It’s very simple to put the airplane seatbelt through the back of the car seat and we felt he was safer (and easier to keep contained!) in a car seat. I remember reading not to check a car seat as it is so common for them to get damaged. There’s a lot of videos and tips from flight attendants that I found useful. I’ll try and find those links and post them.
Yes Cosco was the brand we got too!
The plane ride will likely be stressful, but with extra hands, the trip will probably be nice. Do as few connections as you can, and not super short or super long connections. I’m taking my 23 month old on a trip this summer with my four-year-old. I took my then 20 month old on a 10 hour flight to Italy years ago. Things were definitely easier on the one flight leg where she was sitting in a car seat in her own seat, but I don’t always have the money for all that. Several of the flights involved her sitting in our lap, restless, fussy. She screamed the entire three hours of our connecting flight home because she wouldn’t fall asleep and the time change was killing her. It is what it is. We still had an amazing memorable trip. We brought an umbrella stroller, and the COSCO lightweight car seat. We sat in restaurants, the same way we do here at home - louder restaurants with limited expectations, coloring books, toys, sometimes screens. The flight we gave Benadry it had the opposite effect and she was wilding out. Check their reaction to Benadryl before the trip.

Cosco Kids
Scenera Next DLX Convertible Car Seat
Super light, cheap, great for air travel, but outgrown fast.

Cosco Kids
Scenera Next Convertible Car Seat
Portable, affordable, quickly outgrown, airplane buckle discomfort.

Joie
Mint Latch
Easy baseless LATCH install, lightweight, but quickly outgrown.

Nuna
PIPA Series
High-end travel, easy baseless install, but very pricey.

WAYB
Pico™ Portable Car Seat
Ultra-light, compact travel, quick install, but expensive and outgrown fast.

Ranked #1
Cosco Kids - Scenera Next DLX Convertible Car Seat

Ranked #1
Cosco Kids - Scenera Next Convertible Car Seat

Ranked #1
Cosco Kids - Scenera Next DLX Convertible Car Seat

Ranked #1
Doona - Doona Series

Ranked #1
Cosco Kids - Scenera Next Convertible Car Seat

Ranked #1
Nuna - PIPA Series