We consider ourselves frequent cruisers and never cruise without our kids. My kids took their first cruise before they were one !
Cruising with a four year old and a one year old doesn't mean you can't have fun!
First off: Be prepared. Bring plenty of wipes, diapers, antibacterial wipes, and sunblock. Other things to bring are spray bottels to cool the kids down bottled water & cokes(this will save you money), hats, thermo cup to keep thinks like milk hot/cold (if you have toddlers that drink milk, and extra pacifiers.