I used to use them on my young (preschool) potty-learned kids for airline travel. As soon as DS became emphatic about it, I didn't push the issue. My reasoning for having them wear them, even after "trained" though, was that trips to the potty tend to be the "I have to go RIGHT NOW mama!!!" type - and that's never a good thing if it's take-off or landing time.

Or waiting in line for take-off, then a delay, then waiting, then the take-off and the waiting for the seat belt light to go off....you get the drift.

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When I had a young (preschool) child who didn't protest, I opted for a pull-up as a back-up. We'd keep undies in our carry-on to put on when we landed and I'd tell them that the pull-up was just because there were some times when it is a saftey rule to stay in our seats and buckled and those times may be longer than they could "hold it." Hasn't been an issue with DD because we haven't traveled recently...