Most partitions on the Dream, Fantasy, and post-refurb Magic can be opened. The exceptions are about half of the partitions on deck 10 on the Dream and Fantasy can't be opened because there's a solid structural support between the rooms, and on all ships there are rooms on the angled sections in the "corners" where there's just not enough room to have opening partitions.
No partitions can be opened on the Wonder, to the best of my knowledge.
We were on deck five aft (port side) on the Fantasy and had our room steward open our partition between ours and our daughter's room. It was great! I loved having my granddaughters knocking on my "back door" in the morning! The rooms were 5152 and 5154.
GET UP TO A $1000 SHIPBOARD CREDIT AND AN EXCLUSIVE GIFT!
If you make your Disney Cruise Line reservation with Dreams Unlimited Travel you’ll receive these incredible shipboard credits to spend on your cruise!