No, you're not being petty at all! You bought them because according to the description, they were the right size for your son. You now can't use them because a) there are no inserts and b) they (or one, anyway) are the wrong size. They are in the wrong as they have not described the item correctly.
I would contact the seller and explain why you are not happy and see what they say - if they are a decent seller then they should hopefully offer you a refund. If not then you can start an 'item not as described' dispute through Paypal to get a refund (although you will need to return the item first, you should send it recorded delivery as you will need to prove that you have returned it).
Just out of interest, are they one of the 'regular' nappy sellers (the eBay cheapie pockets, etc?) or is it just a person selling some nappies? If that makes sense, lol.