We booked 4 out of 5 of our Disney cruises late within 75 days of sailing, and everything and I mean everything was sold out online. We always were able to get tasting/mixology classes, etc. For example, on our last cruise this month - about 5 days before sailing mixology became available, the character breakfast and some excursions that were previously sold out.
Even when they didn't before, we were able to book on board. Our DS wanted the pirate makeover and all online spots at BBB were taken with the exception of some 11 am and 8 pm ones. When we visited BBB Boutique once we boarded, they had soooo many time slots available.
The only place where I experienced lines after lines- was the non ticketed character greets.
First couple cruises we stood in lines for 2 hrs each day waiting on the different characters.
Now we only join if there happen to only be a few people, and otherwise get our character photos during the character breakfast and ticketed meets. I found our cruises were much more relaxed not chaising and waiting for characters.
Same with lines/crowds at the buffet especially on sea days. This past cruise we did a lot more sit down breakfasts and lunches - so relaxing and uncrowded.
Excursions and pools - we experienced this once when we had one in our travelling party under 3.
There should be some selection of excursions for all ages, maybe a beach or resort for the day were your child in diapers can happily splash away? Also, we found one excursion where Disney required the child to be 5, but the same tour independently didn't have an age limit.
It really depends on the younger child, if it's a baby under 2, he/she should be easily satisfied playing in the toddler splash area or sitting watching funnel vision. Ours at the time was 2 months away from being 3 and in diapers, so he was very aware he could not be in the pool. I will admit that sucked. But you already have the advantage of knowing this ahead of time. Create a plan that you think will work for your family.
Having traveled with a 2 almost 3 year old, I do think some of your fears are funded. But try to relax, you should get your stuff once you board, if you don't before and believe me -
DCL will make it special for your baby and your family.
Watch the personal navigator, they have lots for toddlers. Wake up with Disney Jr, toddler time, baby diaper race - there should be lots. Granted I have not been on the Dream but they should have baby/toddler friendly activities.
Visit kids clubs during open house hours.
You will have an amazing time.
If however you are still apprehensive, move your sailing to a later date if you can. When both kids will be able to use the pool etc. Go to a resort now instead.
My friend did that because she knew they wanted all kids to be able to use the pools.
I would personally still go, but you know your family best
