Hi NYCmom,
Congratulations on your new mattress first of all … and I’m looking forward to your feedback when you receive it 
I’m sorry to hear about the mixup with the pricing but based on my conversations with them they are certainly knowledgeable and will provide you with accurate information about their mattresses.
Since waterproof or at least highly water resistant is important it will really boil down to two options.
The first of these would be a thinner membrane type of protector that is waterproof and less costly but the membrane is also less breathable so it may interfere to some degree with the properties of any wool in the quilting of the mattress. There is more about these and some links to different versions and options in post #89 here.
The second option would be a wool puddle pad used on top of the mattress with either a thicker sheet or an additional stretch cotton mattress protector on top to help wick away, spread out, and absorb any accidents so that there is a lower risk of any liquids penetrating the puddle pad through to the mattress. If you go in this direction I would make sure you talk with the retailer or manufacturer about the thickness and how much protection the one they sell offers and for a child that isn’t wearing a diaper I would tend to choose a thicker one that is better at absorbing liquid vs a thinner one where the liquid may go through to the mattress over the course of the night if the accident isn’t caught early enough to remove it before it penetrates. The tradeoff is that these are more costly but they retain the benefits of sleeping on wool with good water resistance.
Phoenix