In case you missed it, I have four children :) I have been in your shoes a time or two or four! That being said, my youngest just turned 3 years old. I know that there may be some new things on the market that were not available 3 years ago, but I think the list of items below stand the test of time.
And honestly, you probably don't need half of the amount of stuff that you think you do!
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Pack N Play
Pack N Play sheet
Extra carseat base