Eternabond is the way to go, you'll be glad you didn't choose an alternate product. Here is a little write-up I did when I installed Eternabond on the roof of our motorhome.
You can buy Eternabond at Amazon. And here is a review I did on it.
Good Sam in itself is not much of a value unless you get a really good rate. We found a card in Walmart for $7 and good for a year. As for your 3 year old, be ever watchful of what he can get in to! :)