I don't think a five year old would do much damage to a subwoofer.
Haha, um, no, a 5 year old can kill a sub easily. I remember when I was around that age, my dad had those big old fashioned speakers from the 70's that had a 12' woofer. I was curious about whats inside the woofer, so I took a sharp pencil and made swiss cheese out of the cone!
As for Emotiva, it is a company run by people, and people make mistakes, I would have a little bit more patience. If you have used the sub, and are satisfied with the performance, you might give them one more chance to make it right. However, with a 5 year old running around, a fabric grill may not be enough to protect the woofer anyway. If I were you, I would have purchased a down-firing subwoofer, just to be safe.