Last sentence- should? Hmm,
DCL felt differently. I go by their rules rather than what someone else thinks I "should" do or not do. had they told me it was impossible and I'd have to wait- I'd have waited. But instead they were oh yes no problem at all!
(and btw- that part wouldn't make a difference anyway because we later found out that booking early or not- we could easily still book anything we wanted for them once on the ship at the first reception anyway.

apparently they reserve stuff like that for concierge guests and do so with enough slots for them to include their immediate family which they've also booked in another room, assuming a family of 5 would want to do excusions together I suppose. I cannot say if they would do that for those just booked on the same cruise as extended family and friends though)
As for the issue of sundeck and lounge, I'm willing to guess that most likely Dream/Fantasy won't have an issue with if the key is switched over from child to adult because adult had to be booked in the other room "on paper" but won't actually sleep that way- they won't have an issue with that. It's still paying for 2 people, 2 people using the concierge areas. I bet they won't care at all- same as they don't care (now on Magic/Wonder) if you switch everyone around where they are actually sleeping and give you keys for such. and especially considering how they've handled concierge up till now. They might not continue allowing the entire family- but I'd be willing to bet they won't mind a parent switching out with child who has their name on the room but isn't actually using the room... as long as the same amount of people booked concierge is the same amount of people using it. And personally- if 2ppl are paid for and only 2ppl are using it, I fail to see why anyone else would care if it was my son or me.. one or the other. It makes no difference to anyone else WHO the 2 ppl are. Would you prefer my son in the lounge and sundeck than myself? I doubt it.. 13yr old boy vs. adult woman.. hmmm no brainer IMO *shrugs*