Maybe get out of the pool by three and have the children shower/bathe before muster. After muster and the sail away party, you should have time to get ready yourself if you're quick. Or let your DH stay at the party with your girls while you get ready. Or let them shower after dinner. A bit of chlorine/bromine at that age is not the end of the world. Muster is pivotal. Late for that and everyone else at your station is impacted. I agree with others that the first day is rather hectic. I would try to simplify things as much as possible. Have a great cruise.