I know of agents that will sell the dining plan as long as a ticket is bought for the infant as well.
You can try that and see what happens. You may still get told no and have to have the infant eat off your plate or buy their a kid meal for them.
But then you are basically telling Disney that the child is age 3 or over - and that may affect room occupancy rules; ie. 4 persons plus 1 child under age 3 in a crib is the standard for most rooms...
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