I just booked a ET HBO fare for my wife, 4 yr old and infant son. Wife and 4 year old are gold. No mention of free checked bag on MMB. I booked myself a 'plus' fare on the same flights which includes 2 checked bags for me as I am gold too.
I find that since the seat selection has become free of charge for gold/silver on HBO fares (and also Bronze at 7 days out), it's better to make 2 bookings for the family (with everyone on a HBO fare booking except 1 adult on a separate booking that includes baggage) in order to save money.