Q - Is $40 worth it for club lounge access (for 4 people)?
A - it depends on your spending habits. If you're the type to go out to breakfast every AM as a family, that may cost you $40 right there. In the evening, if you're the type to have a few beers or drinks at the bar (typical US beers... Nothing too fancy), that could run you $25. Also, it is possible to eat a light dinner in this club lounge as they do have sandwiches and other light dinner items. However, if you're the type to buy cereal at the store or run the Dunkin Donuts across the street for a bagel in the AM, it may not be worth it. It all depends on your typical habits.
This is a good US club lounge. Its not the hugest thing in the world but the selection is good and the people are very nice.
As for Gold upgrades and it being sold out. I can't comment on Gold as I am Plat but I can say that this hotel is a hopping place and they are sold out often. I enjoy this hotel very much. Your kids should too.