We have 3 baby bjorn little potties. One for the car, one for home, one I got for my parents' house and ended up taking to use on our trip home.
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For the big toilet we have a seat reducer that was the cheapest one available at Target the day I was shopping. When she was in her toilet phase, that was great.
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I also have a fold up seat that was supposed to be for the diaper bag, but only got a couple uses since dd preferred to be held over the larger potties. Now that she's larger, she just sits on the very edge of public toilets and I support her hips.
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It's been really convenient to be able to just slide a potty under her wherever she's playing, it's also great to have the option to just pull over and set a potty up on the back seat (there are no good places to pull over going past the loop on I-90/94. Guess how I know.) Another advantage is that she can get it herself and can be left to amuse herself with books and toys and just get checked on periodically.
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Otoh, she does most of her pottying at home, in our car, or at crunchy friends houses, so she doesn't have a ton of experience with having to be in the bathroom to use the potty (and has to basically be shut in to the bathroom with me and some toys before she'll consent to sit down). She also thinks that you use a potty in houses and that big toilets are only for stores, so we pretty much have to carry the car potty in with us when visiting families without toddlers.
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Ymmv, my friend's ds who had/has just as many potties as dd will happily clamber up onto their toilet and use it. I'll admit I'm tempted to teach dd, but don't know how and think it's probably not particularly safe if the kid isn't already nimble enough to work it out alone.