Reddit mentions of mifold grab-and-go Car Booster Seat, Slate Grey – Compact and Portable Booster for Travel, Carpooling and More – Foldable Child Booster Seat Fits into Glove Box and Backpack

You probably need one. [This is a neat one that isn't too intrusive] (https://www.amazon.com/gp/product/B01H5VJJTE/ref=ox_sc_sfl_title_3?ie=UTF8&psc=1&smid=ATVPDKIKX0DER)

Hey! I had a similar dilemma and found this awesome invention. The mifold booster seat. Its a portable car seat that meets NHTSA regulations and can fit in a purse. It's obviously not as safe as a full size seat, but it's perfect for travel or if you want to drop off the kids with someone who doesn't have a spare in their car. We bought one for our last trip to Orlando and my wife just put it in the carryon. I wouldn't recommend it for any long drives, but it's fine for short trips from the airport -> hotel or from hotel -> parks. Super convenient and highly recommended. It'll be cheaper than renting a car seat and you get to keep the seat for future!

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*P.S. I know this might sound like I'm shilling, but I have no affiliation with the company. Read the Amazon reviews as well. :)

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https://www.amazon.com/mifold-Grab-Booster-Seat-Slate/dp/B01H5VJJTE/ref=sr_1_3?crid=2GRIMCM6JRF4N&keywords=mi+fold+car+seat&qid=1557342499&s=gateway&sprefix=mi+fold%2Caps%2C146&sr=8-3-spell

We have been using the Mifold travel booster for my daughter while traveling, and if any of your kids are old enough, they can buckle it themselves. It’s tiny.

Mifold booster seat on Amazon

Definitely no infants, but if they are old enough for a booster seat get one of these:

https://www.amazon.com/mifold-Grab-Booster-Seat-Slate/dp/B01H5VJJTE

It's small and won't take up any trunk space, and compliant with all regulations as far as I know.

Your kid is still too small, but look into getting this in a few years. Awesome for ubering with young kids.

https://www.amazon.com/dp/B01H5VJJTE/ref=cm_sw_r_cp_api_i_GSHqDb8XSGCZK

I think amazon has them and you can buy em off their site. They are amazing cause my son carries his in his backpack in case my husband ever wants to pick him up from school. Soooooo much better than a bulky booster seat.

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.amazon.com/mifold-Grab-Booster-Seat-Slate/dp/B01H5VJJTE&ved=2ahUKEwif-amw_-zlAhVHHzQIHaJLACkQFjAPegQIBRAB&usg=AOvVaw11jv0rT_tlW1VyvnxiQ1lv

I just got a Mifold and it's incredibly small, sturdy, washable and awesome. (https://www.amazon.com/gp/product/B01H5VJJTE/) If the parent doesn't have a seat for their toddler, I'm golden. It won't work for infants or small toddlers but I doubt there will be many parents/caregivers THAT stupid or negligent.

Foldable car seat. Small enough to fit in a bag or purse. Won't work for infants but should be ok for toddlers. They do have to test car seats before selling them so I assume they're safe (enough). Whether that's correct or not, I don't know.

https://www.amazon.com/mifold-Grab-Booster-Seat-Slate/dp/B01H5VJJTE

As far as car seats go we have used a slim booster like this one. But your kids need real seats.

For babies that actually needed a seat (like your kids) we just had to lug the forward facing/rear facing child seat around. Getting the straps that can secure a forward facing seat to rolling luggage was a godsend. You can even have the kid sit in the seat while you roll the bag/seat through the airport. It makes life a lot easier if not lighter.

For rear facing seats we just had a foldable stroller where the seat clipped into the stroller.

Everywhere we have been (Mexico and Canada) the hotels had cribs. We did use a little foldable bassinet like so for some trips. Though I can't remember when the kiddo grew out of it. When the kid was older we just had her sleep in the bed with us.

Some places also have car seats you can borrow. I have friends that rented a car seat/stroller in Europe when they visited. They did infant in lap on the plane and then rented the seat/stroller when they arrived. Every car rental place I know of has car seats you can rent. So if you can avoid having to lug one around it isn't a bad idea.

Also, don't forget that the little ones need passports too.

A tablet for the 3 year old with some educational games or some movies will work wonders.

If you are still nursing I highly recommend just getting one of those nursing blanket/cover things and nursing during takeoff and landing. It helps pop their little ears and keep them calm.

I am trying to think of anything else that helped. It is going to be a production either way so good luck. Have fun. The cool thing is that the 3 year old might remember the trip. My first memories are of traveling in Europe when I was 3 and I always thought that was pretty cool.

Or cheaper than long term parking is this portable packable “car seat”. I’m not sure how old your kids are or if you’d even be open to something like this, but it’s helpful in a pinch:

mifold Grab-and-Go Car Booster Seat, Slate Grey https://www.amazon.com/dp/B01H5VJJTE/ref=cm_sw_r_cp_api_i_.3s4BbSE6GCH6

I agree with all of this. However in response to #2 let me point to this:
https://www.amazon.com/mifold-Grab-Booster-Seat-Slate/dp/B01H5VJJTE/ref=sr_1_1_sspa?ie=UTF8&qid=1524848774&sr=8-1-spons&keywords=mifold&psc=1

I had a pax get in with one of these and I was impressed (username is relevant).
"most portable booster seat ever invented designed for kids aged 4 and up, 40 to 100 lbs, and 40 to 57 inches tall"

This does not help with idiots who want an infant to sit on their lap, but I have had pax new to the area not prepared for Colorado laws (booster up to age 7).

I got a real small one like this link


Even smaller ones exist link