This. My guess is that even being told differently, the GA was simply not processing the fact that the children had their own seats. We encountered many a surprised GA and FA when they learned our son had his own seat. We never had an unpleasant interaction but things along the lines of "you need to gate check that seat" because they assumed he was a lap baby. With so many parents choosing the lap option, agents are probably conditioned to think all/most under 2yos are lap babies.