You can check the atrium every day - they post times of the character greetings every day. On our 4-night cruise - there were 2 major princess gatherings - one was a photo op/autograph session. The other was an autograph session only.
If my memory serves me correctly - the following were on board with us: Snow White, Mulan, Princess Aurora, plain Belle, Fancy Belle, Cinderella (sometimes with Prince Charming) Wendy Darling, Alice and the Queen of Hearts. I'm sure I've forgotten some.
At the princess greetings with the photo ops - the girls go from the 1st princess, to the second to the next etc. it is not a free for all. So while you may be there 20 or 30 minutes in advance - if you are further back in line - it still may take an hour or so before you get to the first princess. The princess's were fabulous - they spent time talking to each guest - the interaction was fabulous! The photographers will take their pics - and by all means - you can take yours too! There are even CM's to hold the autograph books and pens so they aren't messing up the pictures.
If your DD is bringing a princess dress - this is certainly the time to wear it.
For my DD - this was a great event - although she was 9 at the time - so she actually waited in line while we sat in the chairs in the atrium.
Don't let your 1/2 YO DS keep you from doing this.
By all means - have 1 parent stand in line while the other one finds something to humor the kids with.