Breath Canada.... Breath....
That's a large list of questions you are asking there and many of us may not want to try to tackle them all at once, so lets break this post down into pieces.
First question- hotels in Miami - go look in the Port Hotels and Transportation forum. There is good information in there on this topic.
Secondly, go to Carnival's website and read their FAQ. Many of the questions you have asked are covered in there.
I can't answer the young child questions, but maybe someone else with experience can.
What time can you board? That comes up every once in a while and I always say to go early. The travel documents say 1PM, but we have always managed to start boarding earlier. I try to be there by 11, but with small kids, you may not want to do that. Either way, you will be standing in line with the childern. Maybe its best to take the chance and get there early? If you do, at least you may be able to get to stand inside the terminal building out of the weather. Unless they have added to it, Miami's terminal building doesn't hold a lot of people "pre-metal detectors" so the line quickly can grow to the point where you are standing outside in the sun.
You don't HAVE to tip at all. (Not that I'm saying don't tip) and tipping in general is always a hot topic on this forum. But if you think that the $10 a day should not include your childern, then you could have the tips removed from you bill at the purser's desk. But if you are using camp carnival, you should at least tip the Camp staff they work really hard and they ar enot included in the $10 a day "pool". And the room stewards are going to be picking up after 4 people wether they are childern or not. Some may even say the childern will cause more work for them. You will have to use your judgement there.
And the last one I will answer before I let someone else take over, the cabin assignment. IF you booked a specific cabin number and not a "guarantee or To BE Assigned" cabin. Then you MOST likely won't get an upgrade. You can call you cruise agent and tell them you do not want to be considered for a cabin change and that will lock it.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013