We took our DD on 2 cruises recently - in Aug, she was 2.5, and this Jan (she'll be 4 in March).
1) If they are used to naps, let them take one. Happy child = happy everyone else. We get to the port at 10am and board at Noon, eat lunch, and when the staterooms open at 1:30pm, we go in, and it's naptime. The wave phones help a lot, because 1 parent can be off on the ship while one is in the room with the sleeping child. Our DD got to bed later than usual, so napping really really really helped.
2) If you have a strict sch with your child, try to at least keep to the eating sch. Again, happy child = happy everyone else
3) Bring (or buy) a few sand toys - they are also fun in the Mickey Pools (especially a watering can). This attracts lots of other friends, too!
4) Bring something for your child to do at the port while waiting to board - something fun and new that they haven't seen before. This helps keep their interest while the time passes.
Let me know if I can help - read thru trip repots - in fact, we should be posting one for our Jan trip this week (we have on posted for our Aug cruise already).