As platinum SPG member, will I and my immediately family (wife and 2 small kids) be given complimentary breakfast?
contact the hotel directly, usually bebefit is only for member and one companion,you need to check with hotel. Some Japaneses hotel will charge second guest,even children
I too am a Platinum member and the breakfast buffet on the first floor is free. It is a very nice buffet with American and Japanese-style breakfast options. If you are not accustomed to the way that scrambled eggs are made in Japan (they are very undercooked) ask the chef who makes the omelets to order to make you a well done scrambled egg--they will be happy to make it for you!
Get an answer from Sheraton Miyako Hotel Tokyo staff and past guests.