Our DD is 4 and was 3.5 last year on our trip
You register her during the afternoon of the 1st day. You can go an at anytime and sign her up, meet the counselors, ask questions, see the facilities, etc. It's a GREAT way to introduce her to the club with you there.
The club officially opens at 7 pm on the first night and your best bet is to have her there at some point that first night. It's the most upbeat night, everyone is new, they let them run around and explore the pirate ship (huge slide, huge hit with the kids).
They will give you a pager at registration and page you anytime your dd wants you. We never told our kids that, thinking they'd abuse it, but the only time we got close to a page was when Captain Hook was scheduled to come visit the 5-7 year olds. My DS 5 hates him so we went to pick him up. HOwever, we knew he was coming in advance as they give you a children's program guide and it tells you what every activity is and whether or not a character will be there.
Mickey surprised the kids the first night (my DD LOVED this) and the cutest one in the Club (4-7) is when Snow White comes. She talks with everyone, then they sing/dance together - TOO cute - a huge amount of parents showed up to take pictures.
The counselors were EXCELLENT. Just incredible and my kids loved the clubs.
Another tip - find some folks on your cruise (roll call thread) and link up with them. If you can meet at the terminal or at the sailaway party your DD might meet some kids in her age group and it helps for them to have a friendly kid face there when they go in.