I've been in Germany for the last few years, with a family of three (2 adults and now a 13yr old). We've traveled all over Europe as SPG plat, and now recently with Marriott. I've found that so long as I have status, booking the room for 2 adults only is enough. With SPG on arrival normally we'd be upgraded and there would be no fee for an extra bed, but this depends a lot on the properly. I'd say 9/10 times no fee with SPG. We traveled in Japan and China recently, still with SPG, and found the same.
As a plat SPG upgrades were the norm, but with Marriott they are the exception.
With Marriott now things are very different. I still book for only 2 adults, but I always inform them ahead a day or two that we will be three, and ask if we could be 'upgraded to a suitable room'. Sometimes this works, but almost always there charge a fee for the extra bed (30-50 Euro) per night. No extra fee at the JW in London for example.
No fee with kid and extra bed examples (most upgraded). All allowed breakfast for all 3 of us.
Element Amsterdam
R. Dusseldorf (upgrade to 2 double bed room)
LM Stuttgart
Westin Munich
Arabella Sheraton Munich
Westin & each Sheraton in Frankfurt
Element Frankfurt (gave us an extra room no charge!)
Sheraton Heathrow
Bolzano 4P (charged for kid's breakfast)
Venice 4P
JW Marriott London
LM Nice
Design Hotel Rotterdam
4P Dornbirn
4P Zurich Sihlcity
In Asia:
Westin Tokyo
Westin Osaka
Sheraton Grande Miyazaki (awesome stay btw)
Westin Awaji
Westin Nanjing
Sheraton Kingsley Nanjing
LM Sentosa Singapore (onsen room upgrade)
and others
Those that charged an extra bed fee, that I can remember:
R. Tuscany/Barga
Sienna 4P (50 Euro/night even with suite upgrade)
Prague Sheraton (declined the 50 Euro\night, used a camp mat for 5 nights)
Nordwijk Oranje hotel
Des Indes, the Hague (charged for kid's breakfast)
Alas for the SPG days...