If you are open to other brands:
Hilton in Germany will give you a 50% discount on a second room for your kids - which is a great thing.
Novotel always allow 2 adults and 2 children in their rooms.
If you really have to stay at Marriott - these are the ones we stayed at:
Switzerland:
CY Basel - Write to the GM - we got 2 connecting rooms as upgrade (we went there off peak so the GM doesn't mind that)
Germany - room with 2 doubles:
CY Bremen (right by the central station)
Marriott Hamburg
Renaissance Dusseldorf (I was there before their remodelling so no idea if it's still the case but this hotel was very generous in upgrade for elites)
Leipzig Marriott
UK
Leicester Marriott